Sad to hear about this! Though he's no virtuoso, I really enjoy Jukka's drumming, and he's easily the second-most fun member of the band to watch on Showtime, Storytime after Floor (probably also second to Tarja on End of an Era). I have a close friend who really suffers from insomnia, though, so I have some idea of how debilitating that can be and definitely understand his decision to take a step back.
It is good to hear from others that his temporary replacement is excellent, though. I do hope to see Jukka back behind the Nightwish kit soon, though. It won't be the same without him.