I've used it to pay for some camera gear last year before I had a limit that was large enough to cover it. No adverse affects from AMEX. Not the best idea to carry a balance on a charge (especially if you have a revolver), but the option is nice when you need it.
How do you go about qualifying for the pay over time feature?
I don't know if it's because I got a Plat card but it gave me the option to opt in when I activated my card. Didn't give me a limit though...
I think it opens up after a year of membership. Also, there's a hard limit of 35k. APR is around 18% methinks.