Got the Platinum trophy for both Sly and Sly 2, currently ploughing through Sly 3 and am astounded people consider Sly 3 the worst of the trio. I loved the original Sly to bits (great level design, great length, etc), but when it tried to offer variation it fell flat (the Murray race levels for example).
I feel like Sly 2 tried to up the variety but the whole game feels dragged out. While there's a good amount of different things to do in Sly 2, they are repeated way too much. What I did like about Sly 2 was the idea of the hub-world (though the design of these is inconsistent) and the final "heist" missions. I think they really nailed that feel of a cohesive team using their strengths to deal with a particular situation.
That said, Sly 2 was about three hours to long for me so I was worried when I went into Sly 3. But they solved the problem with variety. Nearly every mission in Sly 3 gives you something different to do, and while I can understand people prefering the pure platforming of the original Sly, I really like the idea of tackling different situations with different characters and exploring their abilities. Outside of the main gang, I agree, the "additional" characters are a bit weak -- but they are hardly used in reality.
I think if Sly 4 keeps the formula and structure of Sly 3, but balances it all with more platforming, I'll be supremely happy.
All said, for all the trio's weak points, the whole collection is fantastic. I hope it sold alright and people are playing it, what a great little package.
Can't wait for the ICO Collection. Kinda hoping for Jak & Daxter now.