I use them for everytrip and I always write reviews of where I stay.

Basically I look for a room with a good rating and them only read the negative reviews to see what their issues are.

That way you focus on a good hotel and get any good dirt.

One other caveat - sometimes a review gets posted under the WRONG hotel, espec. if there are similarly-named hotels.
Ex. Hotel de la Porte Doree vs. Hotel Doree, or all the "de Saint Germain" ones...

Brahmama,
For a reasonably priced hotel in the heart of Mayfair, you might want to check out the Holiday Inn there. The rooms are nice and all and all not a bad hotel.

The more posts on tripadvisor, the more I'm likely to trust the overall impression of a hotel. That said, however, I do note the date of the posts, because it can and does happen that 2-3 years ago there were a flurry of posts and then rather a long lull, during which the place may have either been renovated or fallen to pieces. I weigh recent posts slightly more than older ones, but again -- factor in the probability that a post may be bogus or so different from my preferences as to be useless.