>Steam has it's own usability issues though. For example it won't let me play a game and download a game at the same time.

You can do it manually. Activate the game to play, alt-tab to Steam. Library, view the downloads. The download will be suspended, pause it and then you can unpause it and it'll resume downloading. Alt-tab back to the game.

Lots of work, but it works (or at least I think it did when I last tried it).