The negative to this is similar to the negative of Flattr, you're giving a prominent position to a button/brand other than your own for less than the value of that space. On Adam Corolla's site, he's got it in the top banner, a very prominent position. It's only gotten him $3.30* in exchange, a very cheap ad buy.
It will be interesting to see how these perform after a week and a month vs just placing a small ad there. Especially after the $2,000 they gave away to spur usage is used up. I'd wager these will only bring in a small fraction of what an ad would.
One positive over Flattr, though, is at least you can see how much that ad position is paying out as a visitor. 1 Flattr doesn't have a direct connection to dollars.
* - I'd originally forgotten the default tip is 10 cents, not $1. Updated with the 1/10th value appropriately.