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.
Update
Since Patch 1.03 the bug of randomly getting human form back after multiplayer sessions has been fixed. According to the patch notes, you now get some humanity back for helping others as a white phantom, meaning your health penalty will decrease. When all your health is back, you will regain your human form (I have just tested this). This means you are no longer dependent on human effigies to get your humanity back in online mode.
Patch note excerpt:
Players now receive a small portion of Humanity after successfully assisting in a Multiplayer session instead of regaining full Humanity.
Before Patch 1.03:
Before Patch 1.03, you got revived by playing co-op at random. It does not matter whether you defeat the boss or die, sometimes you will return to your world revived, although you placed your summon sign hollowed.
There are several rumours on the internet, that it gets less likely to revive the more often you fight the boss, or that the amount of human effigies you have has an influence on your chances to revive, but they are wrong.
I once got revived although the summoner died in a boss fight, and once it took six times killing a boss and then only one time killing the same boss to revive, although the circumstances had not changed.
This random procedure indicates, that you are not actually meant to revive. In From Software's twitter post, that has been partially translated at reddit, it has been confirmed, that phantoms reviving is a bug and not actually supposed to happen:
Somebody asked the official Japanese FromSoftware twitter if it was a bug that sometimes you didn't revive to human after helping somebody else play coop.
The answer? "Actually you're not supposed to revive to human, we'll investigate why that happens".
Human effigies are limited in Dark Souls 2, as long as you don't use Bonfire Ascetics, which practically turn areas into New Game + difficulty, but as long as you use them rarely and with care, you shouldn't run out of them. You can find a lot of them as treasure and they also drop from (limited) enemies. For more information on that, see How do I farm Human Effigies?.
Should you find yourself running out of effigies anyway, you can either try to find the Ring of Life Protection, which makes you practically immortal as it can be repaired, or you could try to get summoned and ask other players through private messages to give you an effigy.
Best Answer
I don't know what lvl are you, but I would suggest to choose an area that you passed recently. And now, some places where (IMHO) invading is easier:
Iron keep - Iron Hearth Hall: take the small door on the left, kill the Alonne captain and then kill the the two Ironclad on the bridge. Keep going until the large lava room and clear the Alonne archer on the right. If you invade here, you can use platforms and / or fire to help you.
Drangleic Castle - King's Gate: once out the bonefire room, take the first on the right (the room full of sentilens "sleeping"). You can invade here, but do not cross the door on the right or you will invade the area before the next boss.
Black Gulch - Black Gulch Mouth or Hidden Chamber: if you use fire you can set the oil (I guess) on fire (but possibly after the mob came out), use poisoning statues to trap him or other mobs;
Sinner's Rise - The Saltfort: take stairs on the right, than kill the 4 undeads and take the elevator. Once down you can start invading.
I don't know what is your build, but I have few tips if you have trouble invading.
Good luck! ;)