I got maybe a dozen mystery skins back when they were gift only.
My first one was Gentleman chog which was my #1 most wanted skin, so I got hooked on em and kept trading em with friends. Didn't get any more I'd use until I got a firefox Ahri... I wanna say it was the same day that PopStar Ahri got announced though, so that took a bit of the joy out of that one. Wrapped it up with a bittersweet lulu... and of course, now the winter lulu comes out. The other 8 or so I got were one or two skins I'll never use because I have better skins for that champ (Nightmare Chog... yeah, that's never seeing the light of day), or skins I'll only ever use just to be using a skin (Sasquatch Nunu). Would kill for popstar ahri, officer vi or the new nasus skin, but I figure the newest skins probably aren't in the roll.
Basically from my experience, about half of what you get are 520's that were retired because they were just recolors... so in the end you'd usually do better just to buy the skins ya want straight up unless you get crazy lucky and get something like SG Udyr and a legendary you needed out of a half dozen mystery boxes.