I've been very happy with Astrid for about 6 months now. It was a little rough in the beginning, but they push codes pretty regularly. The largest issue I have is that their collaboration/linkages with other accounts aren't that solid. My wife and I share tasks and for some reason, the email notification feature for Astrid for my wife doesn't work.
After fiddling around for it for a couple months, I'm now testing Producteev. The latest version of producteev has a lot more functionality.
Pro: Producteev is on more platforms
Cons: Producteev doesn't have recurring tasks that appear AFTER completion (at least not that I can tell).
Bottom line: Astrid is a great tool if you use it individually. If you want to share, and assign tasks with people not on android but other platforms check out Producteev.