Yes, those loan checks from credit card companies are legit.  However, they can be very expensive to use.  Here are some of the high costs related to using these checks: 

• Generally there is a minimum upfront charge for using the loan checks.  Just for using them, you’ll be charged a high fee, such as 3% or 5% of the check amount.  
• The interest rate may be higher for using the loan checks than for using credit card. That’s because loan checks are treated as cash advances. And cash advances usually have very high interest rates. 
• Generally there is no grace period before interest is charged, so you are charged interest from the date of use. 
• The high fees for using loan checks may only be disclosed in the fine print.  So read everything very carefully and ask others to also read the information if there is something you don’t understand. 
• If you don’t pay your full balance check loan, your credit card company may apply your payments to regular credit card charges before paying down your higher interest check loan balance. 
• You will have fewer consumer protections:
  • Defective merchandise—you can’t withhold payment (as you could with a credit card purchase) until the problem is resolved with the business.
  • If the loan check is lost or stolen you may not have the same consumer protection as you would have if a credit card is lost or stolen. You can be responsible for the entire cost of the loan check, where with a credit card your costs might be as low as $50.

To protect yourself:

• Check all credit card mailings for checks (and credit card offers).
  Shred the check loans or offers before discarding so a thief can’t steal them.
• Ask yourself if you really need the loan, or will you use the loan only because you want something.
• If you really need the loan, look for less expensive loan options before using the loan checks.