They work very well, but a few tips:
1.- Get the "screw-in" bars they sell on eBay: ADJUSTABLE PULL-UP CHIN-UP DOOR GYM BAR HOME EXERCISE WORKOUT TRAINING FITNESS | eBay I use that badboy. It's cheaper and more versatile (and apparently doesn't do much more harm to door-frames that the more "stable-looking" ones). It holds enough weight, too. Don't be fooled, it's a strong piece of equipment.
2.- They CAN dent the frame a little. I have a few light marks from the first few times. Nothing serious, but visible if you look for them. To avoid this, place a thin fabric between the bar and the door.
3.- Tighten to the max. Until you feel like you've just given your forearms a work-out.
4.- Put them in so that, if your grip slips, it slips into "tighten direction" rather than "loosen direction". Nothing ruins a set like stopping to re-tighten a loose bar (or falling with a bar on top of you).
5.- Don't use them just for regular pullups. Put the bar at pelvis-height and do "Australian pullups". Put it slightly too low and hang off the bar. Put it at chest-height and try and push yourself up against it... This darn thing's a multipurpose tool, ya know?! 
Oh, and:
6.- Enjoy your pullups and don't give up if you "sorta" do one or hang for a little and find yourself exhausted. The way they use muscle is weird and there are a LOT of minor muscles to build up, regardless of general or major-muscle strength.
Last edited by Kochin; 02-03-2013 at 05:07 AM.
I rarely talk to people about Paleo/Primal, but, when I do, it's when they're NOT complaining about their weight or health.
You see, I don't really "do" pity. And one thing I've learned about people is that they're more open to suggestion when they're happy and stable. When they complain they only ever want pity, not to be advised.