A problem with Amazon reviews are people give poor reviews due to an issue with shipping. They'll give the product a one star review bitching about shipping without saying anything about the product or wrap up with one line about how product X was excellent. I tend to look for reviews a paragraph or two in length. Anything longer and it's usually someone who has an axe to grind with or unrealistic expectations of the product. Also, five star reviews rarely have any substance...usually just something along the lines of "product's great! recommend to all my friends" which in reality doesn't tell me shit about the product.
I'll use yelp when I'm in an area I don't know. It provides a basic idea of what might be good there and what might be horrid. But, I would rather be recommended someplace by someone who knows the area. The other problem with Yelp is that they allow for businesses to petition to remove bad reviews for being "unfounded." And, I know some places that aggressively do it. There was one bar here in Brooklyn that would do it and then if they figured out who you were would ban you from the bar if you came back. I wrote a bad review for them because my first and only time there it took 30 minutes to get a drink because the bartender was ripping shots with his friends. And, then later in the night when someone left no tip the manager flipped out on him, the customer tried to explain that the service was horrible and he couldn't order drinks all night. Manager said "Well the bartender is a friend of mine," and threw the guy out. Lo and Behold 3 weeks later my review stating this was removed from Yelp.
I forgot about RateMyProf. I used that site RELIGIOUSLY in college and grad school. It was very useful cause most professors don't change their curriculum ALL that often, so if you found reviews from within the last 2-3 semesters, you'd have a good idea of what to expect. I didn't care so much about people complaining about a course being hard, but getting an idea of exam/paper schedules and workload, as well as the professor's methods and temperament, saved alot of headaches.
If a get an interview for a school, I will look into the reviews for that school to see what sort of environment I am attempting to get hired into. I don't really care about other people's opinions when it comes to how I spend my money. There is a take-out place near my house that gets some of the worst reviews I have ever seen because the one woman who works behind the counter comes off as bitchy if you don't go in there often. I very much do not care what other people think of any form of media (TV show, movie book, etc.) Many of the movies people hate (for example, the later work of Adam Sandler) are movies I enjoy very much. I find most book reviews really pretentious, and no review of a TV show has ever been useful to me.
I use online reviews occasionally, including expedia, yelp, amazon and yahoo reviews, but I excercise a little common sense in evaluating them. If a review focuses on only one aspect, or is nitpicky, I disregard it. If there are less than 10 reviews, I take the rating with a grain of salt. If there are generally good reviews, I'll focus on the bad ones and see if there is a common thread to the complaints. For example, I was looking into a hotel that had a bunch of go dreviews but several bad reviews, which it turned out were all recent and all involving non-functioning toilets, due to recent renovations, so I crossed that hotel off my list.
Oh man, after bartending for so long, the phrase "yelp review" still makes me feel weird. On one hand, I think the idea is super helpful; the idea of accountability definitely makes for better hospitality standards. However, people literally lose their job over this shit. If the wrong person's food comes out too slow, or their fork has a spot, or their bartender doesn't know what's in their random ass backwards town drink special, they have the power to call you out on the internet and actually effect your employment. I once had a lady rip my ass apart for having to kick out two of her friends who wouldn't stop drinking liquor out of their own bottle under their table, which is completely fucking illegal. Yes, some people are shit at the service industry and should find new jobs, but some customers are also incredibly shitty at being served. It's painful that more often than not, it's those customers that inevitably end up leaving the most reviews and passing judgement on an establishment or employee. I always hope that people who go somewhere and have a super rad server or experience get their asses on yelp and leave them a five-star. Rest assured, most managers check that shit and it has a pretty serious impact on their job.