I’ve used GTasks, but personally didn’t really care for it or the aesthetics. It’s simplistic, but effective for just that reason. I can’t recall it having any issues with Google Sync, but Astrid also does Google Sync well. I just prefer syncing with Producteev for the extended options.

Astrid supports tags and works within the Producteev Workspaces, GTasks does not. But GTasks works very well for anyone who doesn’t need or won’t use the extended functionality provided by Astrid and/or Producteev =D