I have had my 123d coupe since early 2012. It was only a few months old with 3k on the clock when I got it. Now its on 58k. I do roughly 80 miles a day on my commute (16k miles a year) and average about 45mpg on a tank full. I get about 520 miles out of a tank before I fill up and its not on empty when I do so. Driving style - my commute is about 5 miles of 30mph then 15 miles of nice bendy back roads where I give it some stick, then the rest is just cruising on dual carriageway at 70mph.
Personally I love the car, its been one of the best ive owned. It handles really well with the M-Sport suspension, but if theres more than 1 of you in the car then it can feel harsh. Performance wise, it took me a while to get used to it. I came straight from a 313hp Nissan 350z in which you could really wring out 1st & 2nd and it redlined at 7500rpm. With the 123d, as others have said, in a manual like mine, 1st is next to useless. You drop it in 2nd and you will find that 2nd, 3rd and especially 4th are fantastic. The car really pulls hard in 3rd and 4th which are great for overtaking. It has zero turbo lag and has instant power. Im not one to race other cars and very seldom try to race anyone, but you do occasionally get people having a go and its surprising how many hot hatches/sports etc dont stand a chance. And it does seem extra satisfying that you have left them behind in a diesel. This is actually the 4th or 5th diesel ive had and its nothing like a normal diesel to drive. The engine is very quiet, smooth and much more refined than any others ive had or driven.
Servicing on the cars is cheap & easy. Oil I find it expensive, but the actual service itself is an easy DIY job. Tyres, I replaced all of mine at 25k with OE Dunlop runflats which at the time the cheapest set I could find online came to over £900 fitted. 33k later and ive still not on the wear indicators, although I did do the camber pin mod before changing tyres which has helped with the front tyre wear. When I last checked the Dunlops were down to more like £500 a set, but i`ll probably swap them for something else next time.
Touchwood ive not had any problems at all. The car still looks like brand new with zero marks on the bodywork & alloys. The black boston leather interior still looks brand spanking new, except a few creaselines on the drivers seat from me getting in & out. TBH it looks like its just left the showroom after a wash.
Depreciation wise, judging from current prices & what I paid, i think ive probably lost £2500->£3000 a year on it, which is inline with what I estimated upon purchase. Insurance set me back about 300quid this year. So all in all a cracking car.
However..... if you are only doing 10k miles a year, like others have said, you need to weigh up if a 123d is for you. What i did is knock up a spreadsheet showing all the costs on a yearly basis. Insurance, Tax, MOT, Tyres, Servicing, Depeciation, plus cost of your fuel for 10k miles. Then you can see over 10 months if something like a 130i would be better value than a 123d.