I tend to use gsmarena and wade through the crap in the comments to get a feel for the sort of problems people have.

Another way to assess whether phones have any "staying power" is to see what the comparative 2nd hand price is on ebay:

For example, when considering buying my current phone, I made a short list. The N95 in all its variants was holding its price quite high and relatively few of the phones sold were new or nearly new. The N96, despite being at least better on paper and was a much newer model, was sold more cheaply, and there were many BNIBs and virtually new ones up for sale. It was clear that lots of people had tried the n96, often through a subsidised upgrade, and had problems. When I decided to buy an E71, I found it very difficult to get a nearly new unlocked phone, prices were not far off the pay-as-you-go retail prices, and bidding was fierce!