I've been using Mullers in my Berettas for years and really like them. I've just bought some for my Blaser F3 but haven't tried them yet.
One of the key differences between Mullers and normal stainless steel chokes is weight, they are typically half the weight which tends to make the gun balance a little differently and more pointy (depends on gun and user as whether this is a good thing). They are ceramic coated so they are easy to clean too. Muller claim all sorts of pattern advantages but being a bit cynical I not sure how they can guarantee their advantages given the massive number of combinations of guns, bore sizes, choke lengths, cartridges and so on.
I've never done a proper pattern comparison but the breaks I'm getting seem to indicate that they pattern a bit better than the standard Beretta optima HP chokes with my gun (692) and cartridges and they were good on my 682 too.
I've got Teagues in my Blaser which are giving me great breaks, it will be interesting to see how the Mullers compare and how/if they change the handling of the gun.
ATB Mark