I've loved my Ouidad cuts. I would say at about the 6 month mark I know I need a new cut but I can go longer if I need to.
Carve and Slice should not take off length if you don't want it to. Like it was explained above, you get a base cut first then they go in and carve. So if you don't want length gone just tell them. I've NEVER had it cut down to the scalp and I've had it cut pretty short. I have very thick, coarse-ish 3a/b hair which if dried naturally doesn't hold a curl worth crap, esp, when long. For me I don't know how dry cutting would benefit me.
That being said, with any method you're going to have great stylists who listen and are excellent at their job, and ones that are crap at it. Don't write off a total style just because a stylist sucked. d