I haven't personally played it, and I haven't gone through rigorous calculations like I have for other classes, buuuut:
I've been DM'ing a short adventure series for two players, a Fighter and a Shadow Monk. We've been playing from level 6-8 to far.
I can tell you, as a DM, I'm very impressed. The player's mobility is bananas. He has Shadow Step, which lets him teleport up to 60 ft. as a bonus action. That means he can travel 100ft. and then punch you in the face in the same turn, stunning you. They've been adventuring in shaded jungles and underground mines, so the monk has been BAMF'ing around all over the place, no one can escape his fists of fury.
I'll also say that the player has been getting tons of mileage out of Deflect Missiles too. He's so slippery in melee, I have to try and hit him with ranged, but with a high AC and a reaction to catch arrows, it's really hard to scratch him.
As a person just glancing at the class, never played or math, it looks good. This is not the suck version of 3e, this is more like the badass version of 4e. DEX is your defense AND damage, which is great, less stats to worry about. By level 5 you're hitting 3 times, or 4 times if you spend ki, it's a lot of damage. Stunning Strike is also VERY GOOD.
Also at a glance, Way of the Open Hand looks really good, Open Hand Technique lets you be the 4e monk with battlefield control and it doesn't even cost a bonus action, it's free.
Way of Shadow, I can tell you as a DM, Shadow Step is gold. GOLD!
Way of the Four Elements is what I, as a player, would be most excited about. My favorite character in 3.5 was a swordsage. And yes, I loved the Avatar tv show, sue me. So being a kung fu dude that smashes foes with fists and magic is goddamn awesome to me.
Buuuut, looking at the subclass, I don't think I'd play it. What I loved about Swordsage in 3.5 is that you had so many options. This subclass, you get a cantrip and a spell at level 3, and you're only getting 1 more spell at 6th / 11th / 17th.I like the spells, don't get me wrong, but I'm a man of variety, and I just want more of it. I think the amount of ki points you have to cast them is already enough of a limiting factor. It's a shame.
So wrapping up, I think the base monk is really good, and I think two of the three subclasses are really fun.
Why yes, I am bored at work, how could you tell?