Tripadvisor is great for the photos to get a real indication of the room
The reviews in 99% of cases are great as well, you read the Excellent reviews, then the Terrible reviews and then sort through the ones in the middle
I ignore reviews with 1 post, no matter what
I also ignore reviewers who always give negative reviews to every hotel
Apart from that it's a great resource
Exactly. Use it with some caution, filter out the reviews that are extreme and it can be a great resource. Other sources (such as reviews on booking.com, which at least I think require a booking at the hotel), also can be useful checks. I often read trip-advisor reviews after my stays as well and usually find them quite accurate, and until there starts being significant divergence between my opinions and the "average" reviews on TA I will keep on using it as a resource.
On the rankings - whilst you can usually take the exact rankings with a grain of salt, in cities where I've stayed at multiple hotels, my experience is reasonable correlation between the order I'd put the hotels and the order on trip advisor, and whilst the difference between #11 of 183 and #37 of 183 may depend on the individuals, I am sure there is a difference between #17 of 183 and #175 of #183 is less subjective. Not that I've ever been game to try #175.