I've had one for a few months now. Overall I like it a lot, but it's not without its faults
For web-browsing it really is absolutely top class - it's been able to cope absolutely perfectly with every single page I have tried to visit. Personally I haven't used Firefox as I find the default browser does the job admirably. Connecting to Wi-fi etc is an absolute doddle too.
I really like having the full keyboard and despite its small size, it is very easy to use IMO. One handed texting is out of the question though.
Camera is decent enough. It comes with services to directly upload to Flickr and Facebook and you can download more (Picasa and imageshack are two that I remember)
You can set up native Facebook chat if you're in to that sort of thing.
The screen is really nice - much higher res than the iPhone for example, and the touch is fairly easy to get used to.
The massive amount of internal memory is obviously a major plus point too.
Apps wise - there are a fair amount of decent apps around already and more in development all the time although it doesn't even come close to the iPhone in that respect. They're nearly all free though. Some of them are very impressive though - for example you can get the full open office suite and GIMP running on it. There's also loads of different emulators (nintendo and the like) out there for games. I'm trying out an app at the moment that allows you to use it as a trigger for cameras with IR.
Main con for me is battery life (you'll get a full day out of it but not much more). It doesn't bother me, but I know some people don't like the fact it doesn't support MMS by default (although there are various services you can install to make it possible).
Hope that helps -if you want to know anything about it, I'll try and answer
