Have had a 744T for several years, and the fantastic little machine has served me well, I would find it hard to part with, esp since 664 does not go above 48K (if I record music stuff, I want at least 88 or 96K. 48K is fine for dialogue). Now, I do tons of sound to camera stuff, used a 442 for years, and replaced it with a 552 (even better mixer), not caring much about the internal recorder (no internal TC! No back up recording media!), I always grab my 744T, which interfaces with it nicely. HOWEVER....if I knew the 664 was coming out, I may not have bought the 552. I always dreamed of an integrated mixer/TC recorder to lighten and simplify the bag. While 788T has nicer preamps, it's not set up to be a quick and easy field mixer, is a battery hog, and it's rare I need more than 4 tracks for the jobs I do. Got excited about the Nomad, until non-Zaxcom fans told me how much they hated the non-intuitive nature of the things (I'm talking about very verbal HATRED), and also it looks like such a flimsy toy, and I HATE the XLRs at bottom (horribly inconvenient, in my opinion). Sound Devices stepped up to the plate and may have delivered my dream machine (for the sort of work I do).
Keep in mind, I said I doubt I would sell my 744T, it's a groovy machine I trust so much, more than paid for itself, so don't feel bad about the 552/744T combo. Myself...I have a feeling the gear monkey on my back will steer me into a 644 next year. I'll wait to hear what people think of it.....