I haven't actually done it but I reckon it's probably pretty time consuming. I spent hours doing the monster arena quest and still didn't get 10 of each yet
I think you should try them, but it's not something I'd try and do in one go sorta thing. I think it's a fun thing to play and then go away and come back to, not play it as part of the main game.

I thought I had spent ages getting Break HP's for a couple of my chars, I thought they were pretty lethal but then I went to see if I could beat a dark aeon again and it wooped my ass straight away :rolleyes2

I think I needed to spend a lot longer getting stronger.

I think you should do the monster arena quest I found it fun and there's some real challenges to battle in there, I've only defeated a few so far.

Anyway, yeah you should try it if you want something to do, but like I said just do it over a period of time, will keep you sane, hehe cos it's sooo time consuming, but I still aim to do it all one day
One thing I love about FFX - Getting break hp/damage limit