I like Sleeping Dogs. But I can also understand why it was initally cancelled. Giant Bomb's review hits it on the head. It has some of the worst problems with open world games without really improving upon them. Where it does succeed is its enjoyable story, its great setting, and the brutal fist-fighting gameplay; a welcome change from the focus on shooting of most Open World games. Wei is also a genuinely likeable and flawed character and its always fun to have an Asian protagonist, somewhat underrepresented in games. It's not as pure fun as something like Saints Row 2 or 3 and it isn't the GTA Killer either.
The collectibles and side missions are particularly terrible and not that enjoyable. People criticised Mafia II for having almost no side content and just focusing on the main story; but that was a great story with great missions. You don't need side content to pad out the game if the core is good.
Open world games are often considered too long for a rental, but in this case, if you have a week free and no other games, Sleeping Dogs is a good choice, especially if you love open world games. Otherwise, you're probably not missing out.