It improves on SBK 2011 for sure but not much more.
-Better speed sensation
-Polished graphics (but still old)
-Much stronger front brake (but wide differences between bikes)
-Stronger throttle
-More bikes and many more liveries since it includes four seasons
So these things obviously make the game much better and if you're into motorcyclegames these improvements justifies a purchase. It is a good solid game but it is not the best motorcycle game, SBK 08 is if you are looking for simulation or MotoGP 08 is if you are looking for casual. SBK Generations does something in between. You might not see anything new on the surface for career mode but they have balanced out the upgrading and cut session time to authentic 45min. These changes are small but significant.
AI is the same, they are pretty good overall but not fantastic. They don't bump you too much, run believable laptimes, make occasional unforced mistakes and take different lines but don't struggle much and operate under other physics than you so they go through corners faster with less laydown and such.
Sky and track during rain looks amazing and there is some more spray from other racers' tires. However none from yours and the same forced upon rain physics with changed handling rather then just leaving you with less grip and to deal with conditions yourself which would've been much more appropriate.
Online works fine, just few racers as ususal.
So yeah, good game but I wonder if they will ever do this kind of game with SBK 08 handling and MotoGP graphics. That would be heaven but will probably never happen the more cash-hungry developers become and eager to please a casual crowd that never appreciated motorcyclegames in the first place.