Is it worthwhile to build a competive Standard M15 deck when a block is about to rotate out?
I would seriously consider waiting for the rotation. Two, possibly three of the major decks (namely U/W Control, Mono-Black Devotion and possibly Mono-Blue, whose competitiveness will depend on on the strength of red in Tarkir block) will be completely transformed or outright lost to rotation.

I would focus on getting the key cards from Theros block/M15 that are safe vet to be used in decks after rotation. Stuff like Hero&#39;s Downfall, Sylvan Caryatid, Courser of Kruphix, Stormbreath Dragon, Nissa, WorldWaker, Elspeth, Sun&#39;s Champion etc.