I just finished up the second game. It's a damn good series.
The first one is a solid shooter, but the whack-a-mole game play had me playing it in short spurts, as I found it a bit on the boring side.
Second one is damn good. I played through the first act in one sitting (after meaning to just play a few minutes to check it)... after finishing off the GTA IV DLCs I was in the middle of, I returned to it and played through the remaining four acts in two sittings (starting last night). Unlike the first, it changes things up on a regular basis (always returning to the core game play), so when I started getting a touch bored (like the first game), they pulled something out that revitalized me. Very good game.
1) Whack-A-Mole cover-based combat always flirts with being a bit boring, although they generally racket up the tension enough to keep it tense... in the second game not all enemies hide behind boxes leading to some good combat dilemmas.
2) Some of the new game play modes can be a bit frustrating at first. Two or three times I was cruising along nicely nicely, then the game would throw a change-up at me... cue half a dozen quick deaths.
3) It is a big, dumb shooter... but you can see the devs working their butts off adding in lots of cool touches, especially in the second game.
4) Vehicle sections usually leave you feeling very vulnerable (in most games you feel indestructible right up until you blow up), so there's no emotional disconnect between foot and vehicle levels. Although near the end, the vehicle levels are pure godlike glee, but I think that helps the game achieve a satisfying finale (despite the easy final boss fight).