Reviews written by customers who have bought or used a product featured on a website (and, I suspect, some who haven't) such as Amazon or the imdb. They are, more often than not, used as a form of marketing, since people are more likely to review something they really liked in order to express themselves, hence giving a good impression of the product. This is why almost everything on Amazon gets a 5-star average user rating.

While it seems like a nice sentiment to have reviews written by the general public, the problem is that customer reviews are mostly written by people who don't actually have any idea how to write a proper review. As a result, customer reviews can be placed into several categories, including the following:

Really Uninformative Reviews
Usually consisting of comments like "it's amazing" and "This is the best film ever made!", they tell you very little about what makes the product good or bad or why the person loved/hated it.

Bit-by-Bit Reviews
This applies mainly to albums. The reviewer goes through every single song on the album in turn rather than giving an impression of the album as a whole. This works ok for singles, but is of little use for full-length albums. You can't really describe a song, and it becomes very repetitive.

Life Story Reviews
This will relate the person's life experiences to the product in question. "I read this book at a turning point in my life and by the time I had finished it I had decided to change my name and follow my life-long dream of becoming a wandering minstrel", "By the time I had got to the last song on this album I was reduced to tears and couldn't move for the next three days", or something. These can be quite gushy and painful to read, and are still often not very useful in trying to decide wether you yourself would like the product or not.

"I am in a better position to be able to judge this product than any of you" Reviews/ "I should know!" Reviews
Fairly self-explanatory really. The latter includes comments like "Even though this is a really old film, it still appeals to a younger audience, I should know, I'm only 13!".
A good example of the former is a review on the imdb of Kids, in which the reviewer states that the film must be totally unrealistic because they live in New York, where the film is set, and have never seen the kinds of stuff the film depicts going on. This, of course, makes perfect sense.

Reviews by people who are clearly out of their depth
On every remotely unconventional film or experimental album there will almost definately be a review where the reviewer states that the product is "art for art's sake". Without wanting to sound pretentious, this is almost always because the reviewer has misunderstood whatever it is, or didn't get it at all, or doesn't like it themself. Occasionally the reviewer has a point, but then you find this on anything and everything slightly artistic, so it is to be taken with a pinch of salt.

Really Pretentious Reviews
Just about always written by people who want to appear more intelligent than they really are. The reviewer will namedrop and try to use clever words to try and make it sound like they really know what they're talking about, and that therefore their opinion is more valid than yours. Sometimes the review will concentrate on an area that really isn't important, but allows the person to show off what they know (see the review of the 24 Hour Party People soundtrack on for example).

and finally... customer reviews that are actually useful
These are few and far between but they do actually exsist. A good review typically says a bit about the product, whether the reviewer liked it followed by their reasons why, and maybe what kinds of people may like it or dislike it, also adding in any other useful information (DVD extras, misprints, related products even).