This mode is definitely refreshing after playing so much Ranked Ladder games. Sure, it's frustrating from a competitive standpoint since you have to be ready for pretty much anything, but this mode is probably meant to be more so for fun than competition anyway.
I've heard gripes about adding rewards to Rift Mode (most likely since Gauntlet does this), but you only pay 150 gold for UNLIMITED play, so to me, that seems like a great trade-off.
My main concern with Rift Mode is that I would like to see what cards I can choose from when swapping cards in my deck before having to choose which card to swap. I don't know if this is to prolong the amount of time needed to get the cards you want for your "perfect" deck, and therefore give you more to work for, but I don't think it would hurt to let people see their options before switching cards.