I read a post earlier about people wondering if Axii was used at all.
On my first playthrough i didn't, however i am noticing it to be increasingly if not better at breaking up large groups that Yrden, as well as being one of the quickets methods of killing anyone in 1v1: Axii, walkaround and execute with a strong style back stab.

I've been using Axii on large fights...basically takes 2 mobs out of the fight so I can concentrate on less

My first play I rolled Mage, but didn't level up some of my spells far enough...I think.

I didn't boost Axii, and I don't think it ever worked.

Doin sword spec now. Will play again as caster and pump some of these spells up more to get the total experience. I didn't think I was spread too thin before, but apparently I was.

The reason I never bothered with Axii is because if it worked, it wore off within a few (literally, 2-3) seconds (on Hard). I assume it lasts longer if you bother to upgrade it, but since an upgraded Quen that hits multiple targets was the idiot proof way to go, I never bothered.

It might be amusing to try to do an Axii only run, for shits and giggles though.

I never bothered with it because I was generally juggling other signs, but I use those Fury traps on occasion, which I assume are an upgraded form of the spell, and those things work great in large groups. Especially against specters. Because it not only ties up the subject, but ties up the other mobs around them as they attack each other, giving you time to bomb the hell out of the tightly packed group, or slip in and flank attack stragglers.

First playthrough, didn't know till late in the game that you had to hold the mouse buttom to properly use it, once I understood how it worked, I levelled it up, but found out to be anti-effective. The reason for this is that when Axii is levelled up it grants higher health to Axii targets and damage inflicted by them, but once the sign wore off I would have to deal with X number of enemies that I've just healed. On my second playthrough, I have been using it a lot more than my first playthrough to break up group battles as RuaridhGoG mentioned, but only the lowest level of Axii.

First playthrough. I use the lowest axii level as crowd controller because it will take aggro off of you. I also found that if you get attacked by mixed enemy type groups (say, endrega and nekkers) if you hex one type the other type will keep fighting it even if the hex wears off.

Axii is really strong even unupgraded. Unupgraded it lasts a long time, at least 10 seconds and it takes 2 mobs away from the fight. One under your control and another enemy that he aggros. It's really underrated. I couldn't do some of the battles without it.

Don't flame me. I did RTM, but what is this about holding down a mouse button to get it to work right?

Edit: So, you got me to wondering. Finally got it working.

Gotta be in range, that is a given. And I see what you mean by holding down the button. (Mine is default bound though, but same diff.) Takes a couple seconds to take effect. And it does help a lot.

One thing it allows me is better Vigor management. I can start a fight with Axii, when possible. Roll away, have full Vigor and then put Quenn on. I used to start with Quen, cast something and then not have the regen because Quen was already on. New way makes things go a bit faster.

I am in Chapter II of second play. That is how dense I can be.

Bump, 'cause I have a feelin' this aint obvious to a few other blokes.

Does Axii works on bosses and large creatures like Bullvores?

I don't know about bullvores but I haven't been able to cast it on boss type enemies except the first Arachas in act 1 - if you can call that a boss. I definately haven't been able to cast it on Letho in act 1 for example. And I tried (until I discovered throwing daggers ;))

Well, you might have issues with a bullvore, darn thing will probably hit you before you can get it done.

I did manage to woo assassins and just sat and watched them fight each other, drop an occassional trap or two if needed and just kept a target lock on the one I hexed. One finally killed the other. Some bosses should be immune, others just have a high resistance to it, and you get that annoying backlash.