While nobody seems to have an authoritative answer yet, here's my current understanding, based on what others in the Souls community have managed to suss out:
- Agility seems to increase the speed of your roll slightly. It's also hypothesized to lengthen your roll's invincibility frames, but this can proven extremely difficult to confirm/disprove (even EpicNameBro, the author of the official strategy guide, was apparently unable to get a straight answer out of From Software on that topic). It theoretically also increases the speed of other actions such as how long it takes to drink from your Estus Flask. We know that it most likely doesn't affect spell casting speed or weapon attack speed. Additionally, this side-by-side comparison video (85 Agility vs. 95 Agility) seems to show essentially no noticeable effect on shield-readying speed.
- You get maximum Agility returns from Adaptability up until ~38 ADP. This breakpoint may change slightly depending on your Attunement stat, which also has a very minor effect on your Agility stat.
My personal (educated) conjecture is that Agility is no doubt useful on paper, at least based on what we think it currently does. Even in that case, if you're really good about timing your rolls perfectly, it's possible that you may not particularly need the lengthened invincibility window. The various other effects may or may not be useful to your character, depending on your build and playstyle. For example, a character that focuses on staying at maximum range may not always need the quicker Estus Flasks or roll speed, but heavy melee characters might want all the help they can get in those regards.
In any case, if you're planning on doing a lot of PvP or you just want to be a little bit quicker overall, I'd personally suggest spending a few points into bumping up your Agility a bit (most typically through Adaptability). However, if you're getting along in the game just fine as it is, then you may not need to worry about it.
