abilities
I thought about putting up a table that lists all of Diablos' abilities, but then I decided not to. You can find the table at FFOnline.com, if you want it. Instead, I decided to highlight a couple of abilities that I think really are worth learning and - if you happen to not like Diablos one bit - give you a reason to steel yourself up and go fight him to get these.
First of all, Mug. I cannot stress you how important and useful this is. You can get extra healing items without paying a gil, and a number of items needed for Quistis' Blue Magic and everyone's weapon refinement. Mug needs 200AP, but if you get Diablos as soon as possible and equip him with it right away, Mug should be learned by the time you hit Deling. That's when I always get it, anyway.
Second, if you're like me, you will kiss the TV screen as soon as Diablos learns Enc-Half or Enc-None, because you're either hopelessly lost in a maze with random encounters or are going around the world map in search of extra items. This eliminated all random encounters and in a place with a lot of walking or a time limit, this is....well, it saves you a lot of frustration.
Third, Time Mag-RF. Haste, Slow, Stop....especially haste comes into good use in the game, and it can only be refined through Diablos' ability. Oh, how many times these have saved me!
As the last major plus for acquiring Diablos, if Hit-J. It will do no good if Diablos is with Squall, but for everyone else....you can push the character's accurancy to maximum with this, and with aerial enemies this is a very good thing so that you don't completely depend on Squall.
Sure, Diablos has HP+ (only works in the beginning, imho), Darkside (sacrifices HP for a stronger attack...err.), and Status magic refinement (I was never big on status magic, they never work on enemies when you need them to), which aren't all that glorious (to me, at least) but they're a nice bonus for having him with you.
And what did we learn from all of this? Get Diablos, it'll save you! ^^