I bought one of these kits for my office, applied it as directed, and waited 2 days before marking on it. The mark wouldn't come off. After reading others' experiences, here and elsewhere, I started to get really nervous. It wasn't until I called the Rustoleum helpdesk that I found out what the problem was--the kit was old.
These Dry-Erase kits have a shelf-life of 2 years. Anything over that, and the paint won't mix properly. There's no way to check the date on the box, but there's a code on the bottom of the cans that can help you figure out how long your kit has been on the shelf. (The cans are stamped with a code that begins with a letter and a number; the number corresponds with the year in which it was packaged. For example, T9 or P0 would be 2009 and 2010 respectively.) For this reason, I wouldn't recommend buying this product online. Find it in a store and check the cans.
Another way to tell if the paint is old is to check its consistency. This is an epoxy, so it won't be as smooth as a latex-based paint. However, your paint stick should go through to the bottom cleanly. If there's an inch of gunk at the bottom, it's been sitting around a long, long time.
If you've gotten an old kit (and still have your receipt), you can contact Rustoleum (800-323-3584) for a hassle-free full refund. You can also remove the paint by using an epoxy stripper like
Jasco Spray Paint Remover or
Peel Away.
Now, for the good news. These kits--if you can find one that isn't archaic--work remarkably well. I managed to find a kit with a T0 stamp on it, applied it directly on top of the old kit, and it works wonderfully. I have noticed some colors don't erase as clean as others--red and green give me some grief, and the low-odor Expo markers don't work as well as the regular ones--but that's nothing I didn't experience with my old dry-erase board, and a few squirts of dry erase cleaner takes off 90% of any remaining 'shadows'.