3.5 Pros
It is very tactical, miniatures were pretty much a standard part of the game when I played.
The system is very tight (one or two exceptions such as Psionics), with ability progression fitting alongside level progression quite smoothly.
The books are all available on the internet.
Lots of options that can be added to the game.
Did I mention I had a LOT of books?

It is very easy to pick up.
Huge amount of material for the system including numerous campaign settings.
3.5 Cons
The game doesn't work well at higher levels, (though in a standard campaign this isn't an issue).
Some people don't like the tactical side of things as it does require some thinking and planning.
The large amount of material can seem daunting at first.
The mechanics of the game are fairly obvious, which can lead to Roll playing rather than Role playing.
Personally I love the system and would like to get back into it, probably on saturdays twice a month, but a lot depends on the level of interest amongst this forum.