Well, I average 24MPG general/mixed town driving (with the odd blast), and if I'm careful on longer drives, I can even achieve 40MPG.
Tax is band K so £215 last year IIRC.
Insurance costs me £650/yr at 23 with 3yr NCD and 5 years driving, with my 20 year old partner on the policy too (3 years driving). This is nearly half the price of my old Corsa VXR.
No idea on servicing, as I've only just got mine, but they don't really have any special requirements, interval is 1 year/12,000 miles (Although, I'd always do the oil after six months (3-4000 miles for me) myself)
They are mostly reliable cars, you need to be careful of modified examples, particularly 2007 ones it seems, as they changed the clutch for a short while at some point, and these really can't handle the extra torque.
Driveshafts commonly work themselves loose from the hub, but there was a recall to have them redone with a special sealant on the spines, so you shouldn't suffer from this really, but its worth keeping in mind if you ever need a CV boot doing! (Driveshaft needs to be pressed out of hub).
Other things to look out for seem to be rear gearbox/engine mount, blown oil filter diaphragm and the alternator, but as with any marques owners club, these small trends become easily apparent.
I don't think there is anything common that is particularly expensive, so overall, it should be a fairly trouble-free experience, but like I say, I've not had mine long.