I've been using a menstrual cup for about a year now, and I'm really, really happy with it. I would like to share some of my findings and experiences here, and hopefully convert some of you!
If you're squicked out by female body parts and the stuff that comes out of them, this writeup might contain too much information for you. Then again, if you're squeamish about such things, a menstrual cup might not be the thing for you anyway. Or you might be a man, in which case the whole point is moot.
All the information in this writeup comes from either my own experience, or from women talking about their menstrual cup on the internet. I can't remember all the sites I looked at, but just google "menstrual cup" and no doubt you will find them too. Whenever I mention "women" or "other people", that is where I found them.
Types of menstrual cups
The cup I use is a Mooncup, a silicone version of the Keeper mentioned above. It is shaped like an eggcup on a stem and made of clear silicone rubber. There are several brands that make a similar cup, among them the Divacup and the Lunette. I live in The Netherlands and ordered mine from the UK (long live the internet-credit card combination), but they are available in many places. The original Keeper is also still available, it is made of natural rubber. This makes it less flexible, and personally I don't like how it looks: it's a reddish sort of brown. Then there are some brands of menstrual cups that consist of thin, clear latex with a rigid ring to insert them, rather like a diaphragm. I tried one of these and it didn't fit me, but there are many women who swear by them.
Menstrual cups like the Mooncup come in (usually two) different sizes: for women under 30 who haven't had children yet, and for all other women. I wondered a bit about which size to get, being just above thirty and as yet childless, but I went with the bigger size and experienced no problems.
Getting used to your menstrual cup
When you receive your menstrual cup, its stem will probably be too long. You will probably need to trim the stem to prevent it from peeking out while you're wearing the cup. Some woman on the internet who owns all the different kinds of menstrual cup mentions that she cuts the stems off completely, but this makes removing the cup more difficult. The best thing is probably to cut off a bit at a time and figure out when it's short enough. I left about half a centimeter of mine on, it is useful for grabbing hold of the cup when you're removing it.
Inserting and removing a menstrual cup is a bit more complicated than inserting and removing a tampon. You need to fold it up to insert it, and then keep it folded while you're inserting. Also, you need to find out how far to put it in. If you get this wrong, you will notice: not deep enough and you can still feel it, too deep and you will feel an unpleasant vacuum-like sensation. At least, I think that's the cause of that feeling...So inserting it right takes some practice.
Just like insertion, removal of a menstrual cup is a bit trickier than for a tampon; there's no cord to pull. To remove the cup, you need to insert two fingers into your vagina and grab hold of the underside. If it's too far inside, you may need to push down with your abdominal muscles to push the cup toward your fingers or to pull on the stem to pull it out a bit. When the cup is in reach, grab the underside between your two fingers. You can now pull the cup out. BUT! Do not pull it out in a straight line down: it will not come out because it will create a vacuum. To pull it out, pull the underside to one side, so one side the opening of the cup comes out before the other and no vacuum can form. Careful, don't tip it to the side too much unless you're holding it above the toilet bowl, this could get messy...
Fortunately, you don't have to be on your period to practice all of the above, and because the cup is not absorbent like a tampon your vagina will not dry out from inserting and removing it a few times. Some websites I saw mentioned that it takes lots of practice to insert a menstrual cup. I think I had to try three times before getting it right with the folding and inserting, and then when I started using it for real I experienced the vacuum-y feeling perhaps another three times - so I'd say that the amount of practice is quite reasonable.
If after some practice you're confident about your insertion and removal skillz, you can start using the cup for real. And here's where the fun begins! As far as menstruation will ever be fun, of course.
Daily use of the menstrual cup
For me, the worst part about tampons was always that I never had them with me at the moment I needed them. Sure, your period should be sort of predictable, but mine tended to sneak up on me. Or perhaps I'm just an airhead in this regard. Anyway, the good thing about the menstrual cup is that it doesn't cause toxic shock syndrome and it doesn't dry you out, so you can just insert it when you think your period might be on the way - no need to wait until the flow has really started.
Then, when you are wearing it: no smell! Using tampons I could always smell myself. I've asked my boyfriend about it and he said he couldn't smell a thing, but I used to feel ever so slightly uncomfortable about other people smelling my period on me. Now, no leakage at all, and hence no smell. Very good.
You don't have to throw your menstrual cup away like you do with a tampon, but it does get full after a while and then you need to empty it. How often you need to do this depends of course on how heavy your flow is. I've always had a light flow during my period, and this means that emptying twice a day is quite sufficient: I do it in the morning under the shower, and at night before going to bed. Some people need to empty theirs every few hours. If this is the case for you, you probably have a very similar experience with tampons.
If you do need to empty your cup during the day, you will probably be doing it in a toilet. If you're lucky, there will be a wash basin in the room with you. Then what you do is remove the cup, empty it down the toilet, wash it under the tap and insert it again. Easy. If, on the other hand, the tap is on the other side of a locked door, as it may well be in a public toilet, you can do two things: either use the bottle of water you brought along with you because you're an organised person like that, or just wipe your cup with a bit of toilet paper and re-insert without rinsing it.
Every once in a while your cup will need proper cleaning. Silicone cups can be cleaned with perfume-free soap (the only thing I have that comes close is detergent for Goretex clothing) or sterilized in boiling water or a sterilizing solution. I do this once every month, at the end of my period.
Pros and cons
The writeups above already mention many of the pros of using a menstrual cup: it's better for the environment because you produce less waste, after the initital investment it's cheaper than other products because it lasts for years and it's better for your health because you introduce no bleaching chemicals or cotton fibers into your vagina and you run less (or even no) risk of TSS. Also to me it feels cleaner and I no longer have several partly-used boxes of tampons lying around because of poor planning skills.
A con of the menstrual cup is its price - it might be cheaper in the long run but not everybody can afford its price all at once. Another con is the fact that using it requires a bit of preparedness. If your period starts and you don't have your cup with you, it's not much use. Also in some places (rock festivals and campings come to mind) properly cleaning your cup might be difficult or embarrassing. Another thing I've noticed is that sometimes, when I need to pee and the cup is in a certain position, it seems to partially block the flow. This makes th eppeing take longer and gives the feeling not not toally emptying the bladder - which I've heard is a bad thing. The solution to this is to temporarily remove the cup and replace it when you're done. Finally, if you're a virgin and you wish to remain so, the use of a menstrual cup might be problematic depending on your definition of virginity.
One thing that I've seen mentioned as a pro of the menstrual cup, but might just as well be seen as a con, is that you come into contact with your menstrual fluids. New-agey feministy types see this as getting in touch with your femininity and a general Good Thing. Well, the coming into contact bit is certainly true. When your blood is no longer absorbed by tampons or pads but instead caught in a cup, you get plenty of chance to study it if you wish. And while you no doubt already knew that it's not blue and watery, it was new to me that it's not exactly all fluid either. I think that's enough information right there. You do get to know more about your body this way. Whether that's good or whether you prefer getting in touch with your femininity the shoe-shopping way is something best left to personal choice.
All in all I really recommend the menstrual cup to anyone fed up with the expense and messiness of pads and tampons. If you have any further questions about it, feel free to ask me via /msg.