The MAC 217, in my experience, is heralded to be one of the best blending brushes. Recently, Sephora Collection's PRO line of brushes came out with a pretty decent dupe!
Head past the jump for more comparison photos!
L: MAC 217
R: Sephora PRO #27
Sephora PRO #27: Oval Blending
Top: Sephora PRO #27
Bottom: MAC 217
My MAC 217 is softer and the bristles have more movement to them. The Sephora dupe is still soft but it is slightly stiffer as well. This could be because I've had my MAC brush for 4 years (give or take) and I just purchased my Sephora one yesterday.
Both are made of white goat hair (which I love and hate...love because they're soft, but hate because they stain easily) and both are firmly attached to the ferrule and the handle respectively.
Both are domed oval shaped with the MAC 217 being slightly more full in the middle and the Sephora being a little more tapered (only slightly moreso than the 217).
It will be interesting to see over time how they perform against each other and I will let you all know how I feel about it after I've had some more time to play around with it and break it in.