Although I am quite some time in SL (over 6years) and do know the old viewer ui I must say I absolutely love Firestorm.
It has so many features and is very customizable. I do many different things in SL: exploring, shopping, chat, build, script, making pics, etc. and Firestorm really has a lot of features and tools which are very useful for all of these.
I must admit though that I can't really compare all the features to other viewers as I haven't used any of them for years.
But some weeks ago I found a viewer which really sounded interesting and hasn't been mentioned here so far: Exodus
The only reason why I didn't give it a try was that it doesn't support RLVa anymore.
Does anyone out there use Exodus and can share his experience?