If you follow the instructions TO THE LETTER, and coat every bit of the inside including the inside of the filler-neck and the pet*****-nipple, it will last. I have a tank that was Kreem'd over Seven Years ago - absolutely NO problems with it. Even if I have trouble THIS year, I'll just get another kit and recoat it.
That said, on a tank of more than 2-3 gallons, you might consider an extra bottle of the sealant (even though they advertise a kit will do a 4-gallon tank, I would have liked to make the coating a bit thicker and would have bought an extra bottle if I'd known).
There are other products as well - I've had friends use POR-15 with excellent results (if you FOLLOW THE DIRECTIONS!! dammit!)
POR-15 is a pretty good tank-coating system as well. It is less well known than kreem, but its users seem to be happier with it. Also, if you are looking for a non do-it-yourself solution, most radiator repair shops can coat motorcycle tanks.
I have used "Kreem" with mixed results. Results are dependent on how well you follow directions and how meticulous you are. I score low on both counts so I now send my tanks to a radiator repair shop to coat. There's one in Pennsylvanisa that specializes in tanks ; they advertise in Walneck's. Some local shops will also do it...check around.