The 645D ist a good general use camera. Unless
- You need very fast AF; especially AF-C.
- Want a small camera
- Need a very wide anlge lens. 35mm is affordable, 25mm ist expensive, but nor really as wide as 12 or 14mm on a D800.
I am using the 645D for many things. But there are some exceptions. My small digital camera is the K-01, especially in the city it is much less obvious that I am taking photos, than doing the same using a 645D.
People photos: I don't really like taking people photos, and I am not good in just doing one click and catching exactly the right fraction of a second. Thus, when I have to take photos of people, I take a lot of photos, and my 645D is simply too expensive for this, as the shutter is only designed for 50k shots. Also, in some cases, I needed better AF-C. Not often, but sometimes it happens. I cover people photography and fast AF-C by an old D2x.