Grargar: The first game is a very good one. The sequels, less so.
Haven't tried the last one, as it's probably no good, being made by new developers.
First one is a bit repetetive, but still great playing experience. I think I have played it through at least six times (3 times per playthrough, once on each difficulty level x 2). Definetely a classic, even with some faults.
Second one is the hardest thing to approach. On one hand, it finetunes the game mechanics and improves the experience from the first game. On the other hand, the "love noir" thing is so bad writing and so implausible that it's somewhere between embarassing and disgusting to follow, also it doesn't have that depressing feeling that the first game had. Still, overall, it's a fun experience if you can block out Mona Sax from your brain.
I think this series could really benefit from "back to the roots" reboot.
Keep what is essential, leave out the rest.