I've used Fast Water Heater twice over the past 10 years (once in my old house and now in my new house). Both times I've been happy with the results.
The first time around, I really couldn't have been happier. I used to actually take my friends to the laundry room just to show off the plumbing job (yes, I'm a huge nerd). The only two small detractors the second time around were that:
a) My water heater managed to break on the same day that Fast Water Heater was relocating from Kirkland to Bothell. Their phones and computers weren't working, so it ended up taking an additional day to get the new heater installed (whole days without hot water are never fun).
b) Installation cost a little bit more than I expected, and the new setup isn't "a thing of beauty" (like the first one I used to show off to my friends), but it's functional (and they had to work around some screwy existing plumbing).
I'd definitely consider using Fast Water Heater again and recommend them to my friends.
more