Friend introduced me to this at the weekend. It's a new cab firm and they give you £20 credit for signing up (probably works out cheaper in the long run than buying adverts everywhere), so my cab back to the depths of South London that usually costs me £35 was only a fiver as their fares are cheaper (which works out cheaper than the 3 night buses I woulda taken).
Worth a punt, even if you only end up using it once http://uber.com/invite/z65he
Be curious how good they are outside of London (I know they're massive in San Francisco and I suspect they'll have a few cabs in Manchester and stuff). Apologies if this ends up being a LME only thing. The guy who picked me up had a Prius and said their cabs are cheaper because they use a lot less fuel. Makes you wonder why all cabs aren't more fuel efficient. I was just drunk enough to find chatting about cars interesting.
p.s. whilst we're on the subject of offers, just got two Cineworld cinema tickets for £11 via http://www.tap4offers.co.uk and am on the hunt for more bargains - I blame that coupon obsessed kid on The One Show a few weeks back who got £300 worth of shopping at the supermarket and they gave him 5p change or some craziness like that. Coupon me up, yo!