Though HongPong and Lacuna have valid points, I think the issue is more complex (along the lines of BlueJayW's concepts). What kind of mental distortion one recieves from marijuana largely depends a number of factors.

Firstly, the type (schwag, midgrade, or dank) of weed is very important. Lower quality weed will influence someone much more like alcohol than real weed. One tends to get stupid, giggly, hungry and clumsy. My theory is that this is due to higher chemical content and other impurites that are hard to detect. Higher quality pot will produce more of a buddha-like state of contentment. While you are not exctly as sharp as you would normally be, it does not impair your ability to act normally. One can reason rather clearly, perform complex tasks, drive, code, etc. Any ideas that one might conceive while high on low quality maryjane will quite possibly be retarded, much like most ideas that one might get while drunk. Conversely, indoor pot can get you to see angles that you normally wouldn't.

Another important factor in how marijuana affects you is how long you have smoked for. In my personal experience, the longer you have been a pothead the more normal you are while high. I can personally smoke several rips off a 2 foot bong just before going to a Political Science discussion and get A's on every quiz. Meanwhile, a pothead in training would probably not fare so well. The reasons for this kind of difference are quite obvious. Someone who has been drinking for a while can function much better than a relative newbie after 4 or 5 beers. While they are both drunk, the experienced drinker knows what to expect and how to compensate for the alcohol. The same goes for you when you are stoned.

Personally, the first time I smoked, I had the privilege of smoking a joint of indoors. I got high as a kite, but my thoughts soon turned to my C++ pet project. I was coding an mp3 player for the past month, and had hit a dead end with my playlist structures. After about two minutes of stoned reflection I came up with an elegent, simple and workable solution to my problem. I credit it to being stoned, and looking at my problem through "different" eyes.. Just my two cents