I started playing my Sly HD a couple of days ago. I've played Sly 3 before, but that was a long time ago, so I only remember parts of it. But I have to admit Sly 1 was extremely good, even if the gameplay differs from Sly 2 and 3. If you are looking for more background on the jobs then Sly 2 and 3 are definitely what you are looking for, but despite that Sly 1 really has it's own awsomeness. Long story short. They are all really good and are definitely worth looking in to.

I don't get how the games are childish. They are fantastic games, especially if you love that stealth mentality, although it's not that much stealth compared to something like Metal Gear. The characters and settings are what draw me in, along with the individual stories of each heist, especially during Sly 2 and 3.

That's the impression I have - I love the Jak games but Sly seems much more childish