The Mix of Macs at home is growing:

The researchers note to that while Apple ownership is growing, the mix favors mixed system environments. Of those 12 per cent, nearly 85 percent also own a Windows-based PC.

In my experience, Macs plug into an existing Windows infrastructure pretty cleanly - at least so far as home computing goes. I don't do anything with domain servers, so I can't really speak to that. But on the home level of sharing printers and drives, it works great.