Yes it's taxable. But differs by company how much they take out of your check for this.
I used to have a company vehicle and they didnt' take anything out of my check and didn't note it anywhere on any of my tax forms. It was like I didn't actually have one of their vehicles or at least they weren't reporting it to the government.
My wife works for a LARGE coporate company and has a company car. They take roughly $80 bucks per month out of her check to cover tax for "personal use of company vehicle". Some time in December they take out slightly more to make up whatever else is needed, but it's usually not more than $120 that month (with the $80 included).
In my opinion, I love her having a company car. They pay for all her gas, vehicle maintenance, tires, brakes, oil changes, insurance, etc. $80 bucks a month is CHEAP to pay for all that. They only thing they won't pay for is having it washed.... darn...
-mike