I have an ICS sig 551 and I'm very happy with it. Shoots accurately out of the box (put a TB barrel and you're laughing) and has a nice rate of fire. The overall fitment and quality of the gun is very nice and the hop up is very easy to adjust. It's a full metal gun but it doesn't weigh a lot only 6 lbs. My two biggest complaints with it are the swinging trigger guard is too easy to swing and can get annoying. I put a couple dabs of glue to stick it in place and I totally forgot about about it. The other issue are mags, it takes proprietary mags and ICS ones can be tough to find. That being said Tm and MAG brand mags will fit and feed perfectly with a small modification see thread >
< The best thing about the Sig 55X other than the fact that it looks so damn cool are the mags. They will lock together (like the G36) so your loading time is virtually cut in half. The other cool thing is that the mags are clear, this lets you easily see how much ammo you have left. I love this gun I find it to be very comfortable to shoulder and nice to operate.