I liked them so far. The first one feels like Jak&Daxter with sneaking and thievery elements. You'll be in one central hub for a level, then you'll go into sub levels and after completing a few you move onto the second half of the level, then the boss. It goes like that for the rest of the game, but they do mix it up with minigames and good platforming. Then after you finish the game, you can get some ingame commentary that's really a joy to hear and time attacks that are hard as hell.
Second one TOTALLY changes everything. You have more of everything. Playable character, power ups, health, ways to get money, mini games, etc. And the main hub changes somewhat. You will play most missions on the field or in a minigame set away from everything unlike the first, and sneaking around is more necessary.
Sly 3 does the same as 2, but improves a couple things, yet takes out a couple things that I liked.
Best thing about them. I spent about 10-15 on each individual one. They were pretty cheap for such good games.
"Cogito, ergo es eggo." A.K.A. "I think, therefore you are a waffle."