Rounded allen bolts are relatively easily removed using an allen driver bit, a punch, a small hammer, and a drill driver.
Basically, put your driver bit in the rounded out hole, use the hammer and punch to "reform" the allen bolt around the bit, then attach your drill driver and unscrew it. The tappety tap will usually unblock the bolt, and the driver gives enough "smack" to spin it out afterwards. That's pretty much the magic bit - whatever you do I'd recommend whacking it first.
You *will* need a new bolt, and if you're doing it on a rear mech, you'll want to support the part of the mech that the bolt threads into, to avoid bending the rest.
"Easy outs" are dogshit, IMO. Otherwise there is the option of getting a pair of mole grips onto the head of the bolt, or slotting it with a dremel or junior hacksaw in order to use a flathead driver on it. putting a pair of jump leads on it for a couple of seconds can help, too, one end on the head, and one on the mech (you *will* get sparks doing this), the corrosion inbetween the mech and the bolt will heat up resistively and "burn" out, which can be enough to make the bolt easier to remove.
