As an experienced fap-conoisseur, I'll offer my critique.
I give this image a 7/10 on fappability.
This is because:
1. Damn nice ass shape
2. Ass and leg proportions and shading are bretty gud
3. I'm really not a fan of the whole "genitals showing through clothing" idea (Personal taste.) and any genitals which are pushed out like that seem like lazy drawing to me. I also think that from that angle with her right hindleg lifted up you shouldn't be able to see that much of the genitals. It looks like plastered an asshole and a vulva onto the edge of the right asscheek, because they are slightly to the right of where I'd imagine the ass cleft to be based on where you started it just below the tail.
4. Previous personal taste point cancelled out by the fact that I love Derpybutt
5. Distractingly strange jawline