Disc is pretty much the only heal spec that won't have a hugely noticeable regen decrease going from Heartsong to Power Torrent. If you're Disc only or Disc/Shadow, it's a worthwhile investment to trade a small amount of mana for a pretty substantial throughput proc. If you play Holy with any regularity, I would hold off until you're absolutely confident in your mana regen there.
Also, it looks so much COOLER. Seriously.