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!)
;^D