After clocking in about 14 hours in the past day and a half (something I haven't done in years), I must say I'm really enjoying the game, far more than I expected.
I doesn't re-invent the wheel (in fact, it lifts lots of different elements from lots of different games), but it does so much and so well, it ends up forming a whole cohesive, fun, game experience.
Unlike other open-world games, the missions seem a little more interesting, and a little less repetitive, probably because you do missions for both triads and cops, so while for the first faction you do the usual running and gunning and taking "x" person to "y" place, with the cop missions you do a little bit of investigative work, and it's a nice change of pace.
Even the non-story missions and collectables are a rather fun diversion. It helps that the city of Hong Kong has lots of chances for "vertical" gameplay: you climb and jump through buildings, construction sites, ...
On the technical front, the controls are pretty great once you get the hang of it (the fighting, in particular, is rather fun, with all the different techniques, finishers and environmental kills), the graphics are lovely (I'm playing on a 4 year old PC, and the game runs smoothly at near maximum settings (only AA and SSAO is not at the maximum setting available)), the voice acting is great, with lots of familiar voices, and there's a kick-ass track selection for the in-game radios (what else can I say, when Warp and Ninja Tune have a radio station each =D>; driving around listening to Plaid, Squarepusher, Clark, LFO, Autechre, Coldcut, The Cinematic Orchestra, or "oldies" from Tears for Fears, Siouxsie & the Banshees, Queen, The Who, and many more is seriously awesome!).
Overall, Sleeping Dogs is, for me, a very nice surprise, and I can't wait to play some more.
It's one of those rare games I see myself completing 100%, including all the little collectibles, because it's just so damn fun, even just driving around seeing the sights and listening to some awesome music, or picking up some fights to drop someone on a garbage container, or on the back of a car.