i have all 3
they are among some of my most favourite games HOWEVER.......
SPOILERS....maybe
i personally feel, after the brilliance of the first, and the adventure, that 2 is a let down when compared, it also goes a bit monkey island with the whole pirate dream gameplay and open ending....
it just feels that in the first one, they took so long to establish these characters and tell a great story with a rip roaring almost epic ride, that the second is kind of an anticlimax.
having said that it is a good game with another cool soundtrack and some amazingly hard and irreverrant (spell?) puzzles.
the third moves into a slightly different style and engine, making the characters a little more vibrant and 3d, you also play gina for most of it and it follows on the story from the 2nd, to round out a few plot points. i do feel that in the 2nd and 3rd they "camped" up the characters a little too much and made them more cartoony in their personalities than the first.
its good, but not amazing.
personally i feel you could play the first as a stand alone game and not miss out if you didnt end up playing 2 and 3.
however, theyre great games and do demand some "assymmetric thinking" with regards to puzzle solving sometimes.
just an opinion.