A few thoughts on these - I've been playing the $2 ones a lot on Full Tilt lately (mostly NLHE, with a bit of PLO thrown in).
The biggest advantage these games offer, IMO, is the $/hr rates that can be achieved compared to a full table game of the same buyin. Unless you're playing a turbo, it's rare for a HU match to go past the third level, so you can play four or more an hour without multitabling.
Re: single tables vs shootouts, assuming we win two out of every three matches we play (a shootout involves two matches - round one and round two), and working with a $5.50 buy-in, winner takes $10 in the single table and $20 in the shootout, I make it the following:
Single-table:
Game 1: Win, $4.5 profit
Game 2: Win, $4.5 profit
Game 3: Lose, -$5.5
Game 4: Win, $4.5 profit
Game 5: Win, $4.5 profit
Game 6: Lose, -$5.5
Game 7: Win, $4.5 profit
Game 8: Win, $4.5 profit
Game 9: Lose, -$5.5
Total: $11.5 profit
Four-player shootout:
Game 1: Win round one, win round two, $13.5 profit
Game 2: Win round one, win round two, $13.5 profit
Game 3: Lose round one, -$5.5
Game 4: Win round one, lose round two, -$5.5
Game 5: Win round one, win round two, $13.5 profit
Game 6: Lose round one, -$5.5
Game 7: Win round one, win round two, $13.5 profit
Game 8: Win round one, lose round two, -$5.5
Game 9: Lose round one, -$5.5
Total: $26.5 profit
Keep in mind though, you've played an extra six matches in the shootout format, so it's taken you two thirds as long again to make that $26.50.
$11.5 * 1.66 = $19.09 you would've made playing single table games.
The shootout still comes out ahead, but maybe not by quite as much as was suggested above.
I've been tossing up writing a longer piece on low-stakes HU SnGs as I've been playing a lot of them lately (with about 11% ROI), would people be interested?