York Doorway Chin Up Bar
As a bit of a biased disclaimer I should start by saying this was my first ever fitness related purchase at the age of 14, and I'm glad to see the York doorway chin up bar is still going strong to this day!
The problem with most of the other doorway pull up bars is that they come with non-slip grips; these may be a bonus for some but personally I find they can be annoying when you want to be a bit more 'versatile' with the exercises you choose to do on the bar. The grips are usually positioned for standard pull ups / chin ups at around shoulder width, but I find they limit where you can grip the bar for wide grip exercises. If these style of grips are more your cup of tea though then perhaps try the Bodymax Doorway Pull Up and Chin Up Bar.
Back to business. Like most other bars of its type assembly is fairly simple. The bar comes with a pair of brackets which you screw into either side of a standard door frame (3 screws per bracket) and then the bar simply slots into these brackets. The bar is telescopic so you should be able to put the bar up and take it down in under 10 seconds with a few twists.
A quick last word of warning for the home perfectionists out there - the brackets are unobtrusive but because they're screwed in they are a permanent fixture on your doorframe.