It essentially can automate your entire rotation, and if you put it on repeat, it can actually automate an entire raid encounter. Record it via the MR key, then put it on nonstop and voila.
It's not superior to doing things manually, but it's immensely useful. My G15 version one is still going strong a good 10 years after I bought it, too.
Plus, it's just damn useful. If you have 3 or 4 oh shit buttons, you can map them to one of your G keys and just hit that when in trouble instead of having to push the actual buttons which takes far more time. You just go in and edit out the delay so instead of the 1/2 to 1 second per push, you can spam them all at the same time which is very useful if they are not on a global timer.
I can't imagine playing any MMO without some sort of macro keyboard or n52. I think it's the only reason I don't have carpal tunnel. You can easily use it with GW2 or Diablo 3 and customize it. I even use my G15 with my controller as I have things mapped to the controller keys via GlovePIE then Macro those same keys to repeat via my G keys so I end up not having to spam so many buttons.