It is a different Client, you can download it here:
There are quite a few people playing there, at least enough for Riot to shut down the registration most of the time. I saw many Accounts with Gold / Platinum ranking, they even introduced the new system there a couple of weeks before so I assume there are quite a lot players there.
I only played normals there so far, even though literally everyone has all the champs, 20 rune pages and all runes. I can imagine it's quite fun to play ranked games there