I got my first BKT in Sept 2007. It reverted back with no problems. It was the "original" formula with the formaldehyde. I paid $$$ for it at a fancy salon in ATL, I think it was the Marcia Texiera brand.
My second BKT I got in April of 2008. My ends, like the last 5 inches of my hair, didnt revert. They were bone straight. While the BKT was fresh all my hair was bone straight. I hated it. But slowly it started reverting, except for that last 5 inches or so. That was a formalydehyde free BKT I got because the stylist I asked to do it was scared of the formulas with the formalydehyde.
The third BKT I got was July of 2009. That was OK Keratin, I got it off ebay. I think it has formalydehyde in it. Anyway my curls were back by December of 2009.
By reversion I mean, unmanageable or harder to manage and straighten. My first and last BKT my hair was always curly, it was just smoother and easier to straighten, it really helped with rollersets and blowouts. I didn't have to use as small of sections as I do now. The second BKT was a nightmare and I'm glad most of it did revert and those ends are almost all snipped out. I only have about 1.5 inches left of bone straight ends.