I use my 645D for all kinds of things. I've had it since Nov. 2010; the more I use it, the more I want to use it.
It's not a perfect system. It is larger & heavier than a DSLR. Aside from the brand-new 90mm macro lens, it has no in-body or lens-based image stabilization nor a very high max ISO, so it's clearly not designed for handheld low-light work. It has no Live View, which would be a useful focusing aid especially on a tripod. It does demand some reasonably good technique due to having high resolution and shallower depth of field compared to most DSLR's.
But it's a very well designed camera for photographers, and a ferociously capable image-maker that produces amazing results in the hands of anyone paying attention to it.
I was a long-time Canon full-frame and APS-C user. But since getting my 645D the only time I went back to the Canon was the first time my 645D went out of service due to me stupidly being caught out with it in a heavy down-pour. The 645D was down for several days to dry out in a sealed box with silica gel packs, during which time I went back to my 5D Mk II and was not happy. My 645D went down a second time last year with a blown shutter, and this time I bought a 2nd 645D rather than be without this camera system to complete my active project at the time.
I shoot it handheld and on a tripod. Mostly available light, a bit with flash. I will shoot up to ISO 1600 (the max) without worrying too much because the noise is quite grain-like and not ugly digital artifacts; 100, 200, 400 are without much concern and 800 is very usable. I use focal lengths ranging from 25mm to 300mm, primes & zooms; mostly FA 645 auto-focus lenses but also some A 645 manual lenses, and even a few 67 manual lenses. I've had it in hot humid Caribbean locations, bitingly cold Alberta Rockies winter locations, scorching hot dusty Arizona desert locations in summer, windy & volcanic dusty lava field locations in Iceland. I use it for landscapes, architecture, macro, wildlife, travel, street, some people work. I don't try using it much for action work like auto racing, sports, flying birds or other similar situations where a high frame rate with a deep buffer is beneficial.
I really like the system, it fits my work very well. It can't do everything, but most of what I want to do it can do. I would expect to retire my 645D bodies if Pentax comes out with a newer model offering a couple of features I'm looking for (Live View for one thing). Otherwise I'm going to keep on using them by preference for the majority of what I do...