I think it looks awesome and bet it's a load of fun to drive at even sensible speeds. The biggest issue I have with it is that, providing insurance it's a major issue since it can be for under 25's these days, there's an awful lot of MUCH faster and more capable cars available for only a couple of grand more than the £15k they charge for the UP GTi.
EG, Ford Fiesta ST, Renaultsport Clio,
As much as a love the looks of the Up Gti and the ethos behind it it'd be hard not to be tempted by the competition. Plus that's before you even start considering what you could buy nearly new/ used for £15k or under.....
But if you're a young driver that wants a fun, cheap to insure and run car it probably fits the bill. Or if you just want a city car with a bit of character it would also probably appeal to a lot of people.