Employee reimbursement for using their own automobile will vary somewhat by employer and sector but most organizations compensate employees at approximately the Standard Mileage Rate set by the IRS or the Privately Owned Vehicle Reimbursement Rate determined by the GSA. These rates are both currently (2012) set at 55.5 cents per mile traveled (though the GSA and IRS rates are not always identical).
Government employees will always be compensated at exactly the GSA rate. Most employers choose to reimburse at the IRS rate since they are able to deduct up to that amount as an expense when calculating their corporate taxes. There are other complex accounting formulas that employers can use.
If your employer is reimbursing at approximately the IRS or GSA rate then you can feel that you are getting a good deal. If you are receiving less than the IRS rate you may be able to deduct the difference on you personal income taxes.