Time for a necro! I just bought Serious Sam 3 a few days ago from GetGamesGo at an awesome price of $12, and I have to say, this is the most fun I've had with a pure FPS (i.e. no RPG-lite elements) in a long, long time. It still feels a bit conservative compared to First and Second Encounter, but otherwise it's a really good throwback to the first 2 games. It also washes the bitter taste of Serious Sam 2 away; all's forgiven!
I mean, Serious Sam 2 wasn't a bad game, but like what SimonG said, it's just very average. It's like, oh okay, look, 3 (clockwork?!) werebulls and a bunch of thingums that shoot stuff. Yay. I didn't feel there was much of a challenge, and there really wasn't much to look forward to other than the new themes for every chapter. The guns didn't feel solid, but most importantly the enemies felt weak. The first 2 Serious Sams and BFE are intense enough that, if you man up and play on Serious difficulty, you'll find yourself moving around IRL as you try to dodge enemies and projectiles in-game.
Now Serious Sam 3, it's pretty much Serious Sam in a new graphics engine. I have to agree with most of the critics though, the game starts off so slowly, it's like the developers are trolling us. Seriously, 4 chapters of wading through a mundane, post-apoc cityscape that looks awfully bland isn't a lot of fun. Also, the minigun-switch puzzles nearly put me to sleep. The distribution of powerups is also very unbalanced. Early on, you're clinging on to a few chunks of health, but later on in the game, it's very easy to exploit the ample supplies of health, armour and ammo (especially rocket and C4 ammo!).
Otherwise, it's just really a lot of mindless fun. Really, once you're past level 4, that's when the game starts to crank up to 11. There are a few lull spots here and there, but Croteam more or less knows what it's doing. I really hate the witch-brides though. I mean, those aren't even fun to fight because you're just waiting for them to show up and pop a rocket in their face, and it gets even more annoying when the glare of the sun gets in the way. Seriously bad idea.