I use the dunes in a stall deck that relies on permanents (eternity, shards, permafrost, etc).
Played against Halfbloods:
So far only a very fast creature rush with ruby dragons accompanied with a lack of rewind power/sundials,
and twice a very early butterfly effect entropy halfblood beat me. In all the other cases, halfbloods died against me, many times effortlessly.
In top50, it is also a winner, except for rare occasions where i couldn't build up the stalling strategy.
In pvp it seems to be good enough too. The dunes are ANNOYING, so many times people just give up against them.
While i use them, i wouldn't mind some nerf or change, because i think it is OP. Sure, you need a rainbow (momentum and quint) at least, plus stalling) to employ this. But i think it is something time rainbows could abuse too well. (So a raise of cost to 4 may be fine). That wouldn't cause time decks such a big blow, but time rainbows (which i usually employ too) wouldn't be so powerful with it.