Things I liked: Operation is identical to the real thing (have used springfield, colt, para-ordnance, kimber). Excellent and rapid recoil and cycling. Light, clean-breaking trigger. Clean magazine release. CO2 or GG option a major advantage, personally prefer CO2 after having used both types of propellant, especially since all my other airguns are CO2 (bb, pellet). All metal a great advantage in realism, durability.
Things I would have changed: Slightly thinner grip - just a tad wide for my hands, which are fairly average sized; the grip is wider than a single-stack, though not as wide as a para-ordnance double-stack. May need better quality control or a slightly more robust design on the gas seal where it contacts the magazine - mine broke after just four co2 cartridges. Cover at the bottom of the CO2 mags is quite large, a smaller design would be good.
What others should know: Pyramydair replaced mine in just a couple of days, very cooperative and professional. Would make a great training device (even says for training use only on the box). Instructions are poorly translated, but diagrams help. The match to the real thing comes in great here - if you have used the real thing, you already know everything to do aside from the very intuitive CO2 replacement.