I tend to read reviews on things I know or have seen already strictly for entertainment value or possibly, if I didn't like a movie, to see if I missed anything that makes me ponder whether I should watch it again or not.
I don't really read reviews on something I've never seen/played/read. One, it destroys any element of surprise. Two, there's so many ways to get people's worthless opinions of things on blogs, forums, etc. that reviews just seem overkill.
Movie wise, I basically read a synopsis on imdb.com and say 'this sounds interesting' then forget what it's about by the time I watch it. It works out better that way.
I am woefully behind on movies that I want to see, so I almost always read reviews after I've seen the movie. I'll pan through Ebert's page every Thursday and if there's a big-time release I usually read but not closely enough to remember.
I just watched There Will Be Blood last week. I read the reviews and read the details on IMBD and wikipedia, but I find that reading people's thoughts on this site is almost as valuable. Not sure if it's an age group thing or waht but I tend to agree with most of what others on here have to say about a particular movie. Surprisingly and disappointingly, TWBB did not have a thread. Man, what a great film.