Even though I've already ordered one, I have mixed feelings about the new X-FIVE Legion. The regular X-FIVE uses a full 5" barrel while the Legion version cuts that down to a 4.4" bull barrel combined with their new 2 chamber integrated expansion chamber. A shorter barrel will lower the velocity probably about 50fps and at the same time decreases your sight radius, although I'm not sure if the difference is enough to worry about.
I do know the Staccato XC has been using a similar setup for a while and has been getting good reviews, but I don't know yet it if I prefer this new setup over just a traditional slide and 5" barrel. A 9MM doesn't have that much recoil, but from what I've heard the XC and it's integrated comp is a joy to shoot and the comp gets it back on target in a hurry.
This seems to be a new feature that Sig is starting to use on more of it's pistols and in general it gets pretty positive reviews.