FYI, I'm not going to use the new client until Item Sets are implemented.

Define "implemented".

Item sets you've already made still work, and you can manually make/edit item sets following the instructions Riot's given. There are a few other sites which have made apps to create/maintain item sets that follow those same instructions if you don't feel like editing them by hand.

I play to win, but by the amount of games I play and the rate that I play them, I play casually. As that sort of player, I'm not going to put that much effort into customizing my item sets.