Personally, I think the gemstone skins have been great. Sure, they are mostly just 1350 quality skins but I still feel it's fun to have something to work towards if you really want to try and get one of them.
I feel the latest one, Hextech Poppy, is a pretty nice addition. I really like the effects and sounds used for it.
Who knows though, maybe they'll create other variations in the future where they have different tiers of hextech skins for different gem amounts? I could see them releasing an Ultimate tiered one that cost like 20 gems or something, and had the full treatment of new animations, voice over, ect.
Or maybe they could try something similar to Dark Star Cho, and release the skin(s) for 1350 at first and then remove them and put them back up for Gems (which they will be doing with his in the future)