Been running one for 5 years. My scrubber works great, but I wouldn't call myself particularly successful at this game.
I've built quite a few iterations of them. You can spend anywhere from $20 to $500 building one.
If you want to buy one, look at Turbos Aquatics. He's refined his design really well over time, and puts out a solid product.
The guy (Santa Monica) who more or less started the waterfall scrubber thing has moved on to a floating, air driven "upflow" design. I didn't have much luck with that on my DIY tries, but I see good feedback on his units.