£6k pa (which is effectively salary and taxed as such - so about £345 a month in your pocket if you're on lower-rate tax, £327 on higher), then 20p a mile. That comes with the restriction that your car has to be under 5 years old and 100k miles. Essentially the allowance covers buying and insuring something suitable, the per-mile covers fuel and maintenance.
That kind of amount, if you're doing the usual company car 3 year cycle, means it'll cover a typical family diesel hatch - Focus, Astra, maybe low-end Octavia / Mondeo - on contract hire or PCP. If you add what you'd pay to run your own car you could get something nicer.
Personally, I bought a year-old Civic which had done a big chunk of depreciating already, I plan to keep it 4 years so the deal covers my work and personal miles with a bit left over towards running my motorbike.
You get tax relief on the difference between your mileage rate and the 40p one, which isn't the same as getting the whole difference (it doesn't add up to a lot really unless you do big miles).
