Deep Dungeon - any good?

[Atoli]

It's extremely fast for leveling, but because everything dies in 1-2 hits, there is no way to actually learn proper mechanics of the class you are leveling.
It's generally fun to do, with traps and treasure and transformations and can be surprisingly exciting and dangerous if you get a difficult match-up (since there are no restrictions in place in terms of roles, so you can end up with 4 tanks on a boss that needs to be burned down quickly, or without a healer on a floor with heavy-hitting opponents.
If you clear floor 50/100, you get potshards, which can be traded in for tier 5 materia, so there's that. And as Seig already said, ilvl 235/255 weapons for your job of choice.

Floor 101+ are generally harder and 180+ can be a nice little challenge for 4-man parties
Lots of nice glamour gear and even a unique mount you can get from there, although it's RNG-based, so that might get frustrating.
Also, those floors can only be accessed by not wiping a single time with that specific party from 50-100, and if you ever fail above 100, you have to start all the way back at 50 again.

So yeah. PotD is very rewarding, not too hard, quick to do and generally fun, but can be really punishing if you mess up.

[-DMR-]

Floor 101+ are generally harder and 180+ can be a nice little challenge for 4-man parties
Lots of nice glamour gear and even a unique mount you can get from there, although it's RNG-based, so that might get frustrating.
Also, those floors can only be accessed by not wiping a single time with that specific party from 50-100, and if you ever fail above 100, you have to start all the way back at 50 again.

So yeah. PotD is very rewarding, not too hard, quick to do and generally fun, but can be really punishing if you mess up.

How do I go beyond 100 in the first place, now I've cleared it?

[Felis]

How do I go beyond 100 in the first place, now I've cleared it?

You need a premade party for floor 100+