I've used one. They are superb at what they do. However I'm 4.5 K:d on either pad or xim with a SPM on cod over 600. Switching between the two shows no change in stats weirdly not by even a 0.01. However if you arent good with a pad it will make a difference even though most kills come from map position, knowledge, timing etc. . My brother does better with a XIM over pad but there's a few things people need to know.
-You cant exceed the max turn speed of the game. If you do it results in wasted movement which feels like you're not turning. However good you get it always seems to bite you in the arse when shot from behind. You need to ride the turn speed cap and every game has a different max speed
-Its not like PC. Its like controlling a pad with a mouse.
-Aim assist can screw you. In COD for instance everyone has a low sensitivity bubble around them to help with aim. Its perfect for the joystick where you hold the stick over time to get on target, but on XIM you actually have
to travel further on the mouse mat to break this bubble. What can happen is you get stuck on the outside of this and pushed away from the target. So you need to develop muscle memory to break in to it, from time to time it will mess with you though.
-If you are truly excellent on the pad you will do better then a xim player. the game is geared around pad and the game doesn't know you're trying to use a different input device. For instance if you get concussed you may as well put the mouse down. You need to peg the stick in that situation and you can't do that with a mouse. (You can assign a button on keyboard that makes the mouse peg but it means if youre trying to shoot someone while concussed you're having to deal with the mouse going from full pegged and then back to normal control. It screws your aim badly)
-Some killstreaks are impossible to use with it, and means you have to pick up the pad.....vehicles are a joke with it. You can set up a different config for vehicles but you will be at a disadvantage here over pad users. In a game like BF you'll have to either stop using vehicles, deal with switching input devices (pain in the arse) or deal with sub par movement. You'll lose that seamless BF flow though of hopping in and out of stuff.
-Its a superb device and I'm so pleased its there for people who will benefit it.
-I'd say its biggest advantage for good pad players is access to more buttons but I'd rather take a scuf for that.
-If you are good with a mouse set up you will feel right at home. Its hard to use Both native pc mouse and XIM. Its best to stick with one. Its not impossible to switch all the time but the muscle memory is very different, and it certainly throws me.
-Biggest advantage equipment wise I own is the astro mix amp.
Hope that helps a little.
(I suck at using a pad 
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