I haven't found Power Shutdown that reliable in the early game. Having to mill cards is certainly a disadvantage, and one that you'd ideally like to have Archived Memories, Jackson Howard, etc. to mitigate. Trashing an early Faerie or SMC for free is certainly nice, and if the runner hasn't scored much of anything risking two mills for a Corroder is great if you can get it-- but frequently I've found myself in situations where either:
a) The runner has random low-cost cards that mitigate Shutdown's effectiveness (Akamatsu, Clone Chip/Faerie/SMC when I'd prefer to take a Corroder, various Virus or Caissa programs with low install costs)
b) The card I want to trash has too high an install cost for me to safely use Power Shutdown without Jackson, Archived Memories, etc. (Yog)
For this reason, I've been using Power Shutdown primarily to supplement other program trashing elements rather than in decks that go for fast advance.
On the other hand, my recent Jinteki: Personal Evolution deck uses program trashing ICE (Archer, Grim), net damage (various Jinteki things) and Power Shutdown to destroy a runner's breaker suite. In that context, I've found Power Shutdown incredibly effective, especially against Criminals, who typically run limited recursion. For instance, in one game at a local tournament today, my Power Shutdowns took out two Datasuckers and net damage took out the third, making it impossible for my (Andromeda) opponent's Yog.0 to break through Chum.
At this point "standard play" from NBN doesn't really incorporate other program trash cards, so I find Power Shutdown suboptimal there-- bad mills also REALLY hurt NBN. It's definitely conceivable that this will prove good there, but right now I've been liking it more as a component of a larger strategy.