I got one of those pull-up bars that screw into the doorway. I was doing pull-ups and it ripped out, and I landed full force onto my shins. One of the most painful episodes of my life.
I have found the 'Door Gyms' that hook on behind a Door Frame are much more secure (and the different handles allow different hand positions/exercises).
Usually those things have a weight limit (my own was listed at 250 lbs...I'm about 180 myself so have never had any problems with it. Though I would say if it lists a max 250 lbs anything over about 220 would be dodgy) so they might not be for everyone here (reading some of the bench weights you guys are pressing I'm assuming most of you are over 6 feet and 200 lbs). Also depending on the height of your door frames they probably don't make sense for anyone taller than about 6 foot tall.
If I ever don't have time for a proper workout I usually go for 100 reps (proper form, dead hang - no kipping...none of that Crossfit shit) in as few a number of sets as possible with 1 minute rest between sets (cycling through the four different hand positions the bar allows).