I had one of these on a different car. They are OK (ish) but my advise is to wait for the official VW one (Saet Mii version is available already), and it will match your Up's trim colour/fabric. If you want one now, get an Armster - I have one in the Wife's Aygo and they are top quality, although a bit more expensive. Just had a look at their website, and their new MK2 version (available now for the UP) looks better than the official Seat (& VW) version.
PS: Scroll down on the link to the 3rd model - that will be the UP.
They are very easy to fit, requiring only a couple of self tapping screws. However, the official Seat/VW one actually bolts (not screws) to the chassis so will be a bit more rugged.
I want an armrest for my Up, which is on order, and was waiting for the official VW one to match the leather(ette) seats, but now having looked at the Mark2 version of the Armster I may just get that and save a few bob.
Thanks for all that info. I shall keep an eye on developments, as I don't get my Up for 6 or 7 weeks. The armrest I got for my Juke was top quality and worked very well. Your point re the handbrake, Spud, is a good one. The rest I had pivotted up and out of the way, which was useful when handbraking a lot. I mainly need the armrest to give me purchase and something to lean on when my ageing bones and useless back make it hard to get in and out of any car!!