I bought the Crown Brush c433 Pro Blending Fluff brush becuase it looked like a pretty good dupe for the MAC 217. I have two of them, and considered having another. I use this brush to blend eye shadow and buff concealer under my eyes. It's definitely my favorite blending brush by MAC.
Putting a four year old brush [MAC, left] beside a brand new brush [Crown, right] and a white background makes the old brush look dirty. Filthy even. I use Parian Spirit brush cleanser for daily brush maintenance. In my opinion, it's the best!
Here are the two brushes side by side. When I picked up the two to compare, I could barely tell the difference between them. The Crown Brush is fuller and equally as soft. Oh, and it's on $6. MAC currently sells their 217 brush for $22.50.