I liked DGT GTD, but ditched it because the sync was not perfect; I was losing tasks every once in a while.
I will post a link here below that details a search I did for a "good" cloud-based and android-compatible solution. I did not explicitly look at Mac apps, because in general they are really bad. I decided that cloud was ok when I was online, and if i had no net connection, it would be enough that I had an app on the smartphone that could deal with being disconnected.
Aside from the info on the link, both Pocket Informant and GetItDone.app are in beta right now with very interesting products. Pocket Informant syncs with Toodledo and the beta I have is just fine. Will be even more interesting to see what PocketInformant Online does.