I've been using the cheap ones from the Cajun Marinade folks...
I have two and they've lasted through four comp seasons. They fit my hand very well so I just keep using them.
The ones from Butcher BBQ are nice and work well, but they don't fit my hand as well.
Todd (Plowboy) had one at his class in Michigan last year that was nice. It was a trigger type that had a hose that he put into a jug of injection. I can't find it around here.
I've also looked at this one...
but it only holds 6oz. so I've have to refill it a couple of times. I guess it's cheap enough to try it out.
I like Knucklhed's design and would definitely look at one of those if I were doing large amount of butts or briskets or whole hogs.