A AR8pro has an ER20 collet. I've heard a lot of bad thing about the MuscleChuck slipping. It only takes a second to tighten a ER20 chuck. I really don't see the benefit.
I was looking at those last night and had never heard of them before...I agree. The biggest difference is you have one tool instead of two - but the muscle chuck has one spot that the tool has to be inserted (in that cap screw) and puts one more fitting (variable) between the motor and the tool.
ER20 collets should be able to get your runout to 0.0004", I really doubt that the runout on the MuscleChuck is even close so I would ask them what they guarantee. Further, ER20 is widely used in industry for a reason and is therefore well tested and has a long track record. They're easy to use too. Think carefully as your results will likely get worse and then you may switch back and end up with a shelf decoration.
I bought some new ER20 collets from precisebits.com agout 4 months ago. They are much better than the ones I got that were from China when I got a spindle. I also thought about a Mustlechuck, but I really don't think it's worth The money they are asking for it. It doesn't take that long to change bits with standard collets.