Well I've only just unlocked it at level 60, so I don't know how well it will perform at 90 (although ACT says it's in top 3 best dps in the game the last time I checked), but so far imo it's pretty, pretty good. I'm going to vomit a wall of text on you, if you don't care about monk, feel free to stop reading right here.

Monk's gimmick is his free flow. Unlike rng based jobs like dancer, monk always* has all six of their main combo attacks, you don't have to fish for procs to use anything. Unlike any other job, monk doesn't have a rigid combo. Sam, for example, has a kasha combo and a gekko combo, you can't mix and match them. Going Hakaze > Shifu > Gekko will completely break the combo. But that's not how monk works, monk thrives on fluidity in combat. Monk has three "1" attacks, three "2" attacks, and three "3" attacks, one of each is aoe. So as long as you go 1 > 2 > 3, you won't break your combo, even if you switch from the monk's version of kasha combo to the gekko combo right in the middle of your 1 > 2 > 3 combo. You can even switch between single target and aoe attacks on the fly in the middle of a combo, something no other melee job can do as far as I know. That's a pretty fun gimmick, but it's not enough. It's a foundation for a good character as long as you add something more to make it actually fun.

Besides that, he doesn't even have a 1 > 2 > 3 > *1 > *2 > *3 combo because you switch 1 and *1 (same with 2 and *2) every other combo, but for the third attack it's 3, 3, *3 - two to one ratio. Plus at any point you can give yourself a buff that lets you use any of the six attacks, which means you don't have to open your combo with a "1". In aoe "2" gives you a 10% damage buff, so it makes sense to start your aoe combo with a "2". On top on top of that monk also has a buff that lets them use three attacks without following a 1 > 2 > 3 combo, so you can do 3 > 3 > 3 or *1 > 1 > *1. And not only it's theoretically possible, it's expected. Both aoe and single target have one form that deals way more damage than other attacks (1 for single target and 3 for aoe).

And as a very nice bonus, SE realized that having a positional on every attack is too much hassle on top of having to worry about every individual attack instead of just alternating between combos like a sam or a drg. In 5.X I've leveled my monk to 50 and I just did 1 > 2 > 3 > *1 > *2 > *3 combo over and over again because worrying about positionals all the time was too much work. But now that monk has only two positionals in their basic combo—same as drg and sam—you can focus on the main gimmick of the monk, the free flow that allows you to think about your individual attacks instead of following a rigid combo over and over again.

Greased lightning lowers your 2.5 second GCD to only two seconds, which isn't that big of a deal in on itself, but it makes the game more fun and dynamic. Another good reason why removing positionals from most attacks is a good thing. Worrying about every individual attack and their positional and doing it in a smaller timeframe was a lot of work, I'm glad it's over.

So with all of that foundation, all we need is a cherry on top to make the job feel good. For sam, it's slowly building up your three sens to use Setsugekka. And thanks to 6.0 and level 60, monk has one too now. Every 40 seconds (which is kinda long, but not that long) you spend Perfect Balance, do three attacks, and then deal massive damage with Masterful Blitz. That's already pretty fun, but on top of that using Masterful Blitz also gives you a "sen". After you get a sun sen and a moon sen, Masterful Blitz deals even more damage. Just like sam, you're always working on building sens to spend it on a crazy attack. So if you like sam, monk might be a good job for you, at least that's how I feel now. And of course there's still free flow and greased lightning.

All in all, I'd say monk seems to be in a pretty good spot now. It might be my favorite dps in the game, at least on paper. I might be too smooth brained for it, there's a lot to squeezing out every drop of a dps potential of the monk job while also avoiding enemy aoes and buffing healers with mantra. And I don't really play a lot of dps because I hate sitting in queues for 15 minutes. I usually play whatever job lets me jump into a dungeon instantly.

All of this only comes together at level 60 when monk unlocks Masterful Blitz. Until then, it's pretty meh. Which is why I said I just unlocked the job at 60 instead of saying I leveled it to 60. It felt like a joke for the first 50 levels when I didn't even have six basic attacks, which is why I put an asterisk next to always in the beginning. And it felt like half of a job between 50 and 59. If you already have a level 60 monk, try it out, you might like it. If you want to level one from the very start then oof, prepare for some pain, it won't be pretty.