So my Crusader was able to clear Greater Rift Tier 35 after a week.
I made a Monk on the weekend and it was able to clear Greater Rift Tier 47 after two days. Balance!
With the Crusader I enjoyed following my own build and doing what seemed cool. With the Monk, I'm fully netdecking it and just copying the build (skills / items / stat priorities) entirely and the amount of damage this thing can do is absurd. Get exploding palm procced on a group of mobs, press the 'Seven-sided Strike' button and watch the nuke go off. Each enemy hit by SsS explodes the palm, each exploded palm applies exploding palm to every enemy hit so it's just a massive chain reaction and things go from 99% health to dead in under a second (well...unless you're doing GRift 45+).
As an added bonus, here's a video of a guy solo levelling 1-70 in under 10 minutes:
Note: requires paragon level 600+ and a full set of level 70 whirlwind barb gear with level requirements removed (which by my calculations would require ~92 greater rifts to be able to level enough "Gem of Easing"s to level 25).