Unlike some other bounty schemes, there are small bounties available. One can look for ages without finding a $50 bounty, so long that boredom would probably set in and the hunt would be abandoned. But coming across the odd $1 or $5 bounty keeps interest going at small cost to Colin and still improves the overall source quality. It's almost like a bit of a game, a treasure hunt, where small pickings on route keep one's hopes up of rich pickings down the road. A game that can be set down and picked up later as time allows.
Secondly, Colin was very fair in judging the bounty to be awarded. It soon became apparent that I could trust his judgement; whenever I thought a bug was undervalued another would come along that might be overvalued. On mentioning this to him, it turns out it was sometimes deliberate; he'd be wavering and thought it fair to even out which side of the boundary the bug fell.
As for the OCD, I boggle a bit at some of those style bugs and don't think they were all me! :-) I do recall sending in some trivial thing suggesting it wasn't bountiful but perhaps should be fixed anyway; Colin generously still gave a dollar. I thought it worth reporting even without expecting a bounty because I think anything that interrupts the flow of the reader, causes them to pause, irked at a possessive "its" misspelt as "it's", detracts from the odds of them spotting something more serious.
Overall, a well crafted and executed bounty scheme. In some ways I'm surprised other companies don't do something similar, even if it was with closed source under an NDA. If the feedback is prompt then the bounty hunter can decide to stop if the bounties aren't awarded fairly enough in his opinion, or the company can cancel, perhaps having shelled out their budget or unhappy with the quality of the bugs, without the hunter having wasted much time since the last bounty.