NotopgearinF1 & all,
I'm currently driving a 2003 / 53 M3. After owning a 3 yr old 325Ci M-Sport for 4 years it was my ideal upgrade.
In my opinion the M3 is a great car, and you really get a lot for your money; both in terms of equipment & performance. Even the standard M3's come with many components which are options on non 'M' models... TV & Sat Nav as standard from 53 Reg, full leather with elecric 10 way adustable seats, Bi-xenons, exterior styling, 18" or 19" wheels, auto digital climate, cruise control and more. The performance is outstanding, and it's an everyday car.
Having owned the car for almost 2 years I've had no problems at all, and the last service came to a total of £350. You are of course going to pay more for tyres than your average car, but if you shop around (www.blackcircles.com always worth a try) I've found Continental M3 tyres (made specifically for the M3) for £160 fitted (rear), and £130 fitted (front). The car consistently averages 26MPG which is good for the engine & performance it packs.
Prices are really attrractive right now, with a 50k 2003 good condition model seemingly selling for £15k - £16k. There are plenty around so you get a great choice and can be picky with what you're after. Always check for BMW Service history, and check the car did get it's 1,200 mile first service (apparently this is important on the M3). When looking for good models I asked the seller to send me a list of the servicing garages with dates, and the VIN / Reg, all of the BMW garages were happy to confirm that they had serviced the car, and also told me if any other work was complete; this was really helpful when buying privately. Also always use a service such as HPI, costs around £20 and confirms the car's not been written off, reported stolen, or has outstanding finance against it.
Basically if you want a nice car, well build, which can eat miles with ease, and bring a big smile when you feel like a blast the M3 is a great choice. Pick up a car costing £43k 5 years ago for a bargain!