Keleseth is near to must-have in Tempo Rogue. I went from 19-10 with about 3 losses, and also had very few losses from 10 to 6. Then I had a few on 6, but after like an hour I was rank 5.
It doesn't happen often that you draw Keleseth on turn 2, but he still wins you quite some games and greatly improves your winrate. I also crafted him for the deck and I'm still very happy with him.