I don't understand the recent trend in this thread to call Temporal Mastery unplayable. Quite frankly I think you have no clue what the hell you are talking about. It has a restriction but that restriction is pretty ...restricted. It is only bad if all of those

- it is in the Top 8 (9 otd) cards of your library
- you have no Brainstorm/Jace and die before you get to use it with one
- you have no FoW to use it with! (haven't seen anyone mention this but being pitchable to FoW is a pretty big upside for such a card)

are true. If those are not the case the card is pretty much exclusively upside which is rare for a card. The upside ranges from free cycle (which isn't exactly upside but oh well), very likely upgraded to a free Explore, possibly a free Relentless Assault, a free PW activation, a free use of all but two lands and any combination of those. So you will usually get something out of the extra turn and possibly much more. Both the free Mana and the free combat phase can easily be game breaking.
From a certain point in a game you just play with 4 Time Walk in the deck which raises the topdeck quality by miles. An upside of the card to the actual Time Walk is that you can cast it at Instant speed via Brainstorm or Sensei's Top, for example during their EoT, which seems pretty devastating.

IMO the card is at least extremely good, but probably banworthy.