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.
Ho boy, I do have several head scratchers. That one was just uncomfortable. No idea where this is from, still weird to see. For those who like Photoshopped goofiness. (mods feel free to resize at your leisure since I'm no good at it)