Players Always Roll helps keep the narrative focus on the pc's, speeds up play by giving players stuff to do off-turn, and generally lets players feel more in control. It also expands the usefulness of meta-mechanincs like rerolls. The downsides are mostly just the inverses of this: npc's can feel like fodder, and the world can feel less real. It also removes fudging dice as a dm tool.
Even if I were to go with this, I would probably allow major npc's to roll, and make tests against them (and pvp) roll-offs, despite the change in odds from this.
Other options include Attacker Always Rolls (easy to grok), All Rolls Opposed (easy to grok, keeps players involved, twice as much rolling), Depends on the Mechanic in Use (what DnD actually does, makes magic more special, fiddly), DM Always Rolls (which could be interesting if all rolls were behind a screen), and Defender Always Rolls (I don't know why anyone would do this).