I got this gun for Christmas last year, though I purchased from Airsoft Megastore... it's impressed me fairly well. It's a cheap but effective pistol. It shoots quite hard, which is both good and bad. It's also pretty accurate, although the long and hard trigger pull detracts from that. It's also easy to disassemble and put back together, much to my relief, though it takes a little fiddling. I'd recommend this pistol to most anybody.
Although I'm listing more cons than pros, the pros vastly outweigh the cons. The KJW Mk I is a good, solid pistol for target practice or airsofting, though one must be careful if using it in an airsoft game, as it's very powerful.
Fast velocity (Also a con.)
Adequately accurate (I can hit a two-foot-wide tree perhaps 100 feet away with it most of the time)
Lightweight (Though potentially a con)
Good, simple sights
Decent gas usage (A little over one mag per fill with King Arms green gas, though in cool temperatures)
Fast velocity (I've been shot with it, and it hurts a lot.)
Long and hard trigger pull (Harder than my friend's double action WG revolver, in fact!)
Will double-feed if the trigger is released without firing (It doesn't seem to cause any internal problems, however, and if you know about it it won't happen often)
Plastic build (Though lightweight, it's somewhat imbalaced)
No way to mount optics (Not a big deal for me, but)
Rating: [4 of 5 Stars!]