Just picked one up two days ago from Impact Guns here in Utah for $459 + tax, but haven't had the chance to fire it yet. First of all, GSG-5 Mags will not work; totally different in size to where the GSG's are too 'fat', both in thickness side to side and forward to back; GSG mags will not insert at all into the mag well, and don't even think about trimming them down. This Walther is about the same weight as the MP5; way better built than the GSG, and yes, it has a swing down lower. Collapsible stock is well built and very solid when extended, unlike the GSG's, but a regular HK MP-5 A2 stock will not fit this gun, perhaps some serious material removal might work, but I don't have the time. Unlike the GSG, this gun uses all standard HK push pins on the forward handguard and the lower; including the A3 stock. I see in the manual that there is a fixed A2 stock available and I'm probably going to order one just because I really don't like the A3 stock rail 'feel' all the time.
The fake suppressor unscrews with help of a tool that is included in the nice padded Walther-marked hard case it comes with. The 3-lug is, well, not really a full 3-lug; when removing the fake suppressor off, what you see for the 3-lug is just about it...really about a half of a three lug. Unlike the P-O-S sights that the GSG has, this gun comes with a much better adjustable-height and windage rear sight, and the front sight is removable and interchangeable with a different sight that also comes as standard. I don't believe you can use a HK rear sight tool on the sight though, but it looks like a MP5 rear sight and not like the GSG's. What's very interesting is the fact that the lower receiver is a well built, totally different than the GSG design, and it looks as if the pushpin holes are located exactly in the same orientation/location as a real HK's lower is (interesting). The trigger pull is MUCH better than the GSG's 'spongy' version. I haven't had a chance to pull the receiver and barrel assy apart yet, but first indication, this gun either has a much thicker barrel, or it is well hidden inside a fake full size painted barrel.
There doesn't seem to be anything between the GSG and this gun that is interchangeable at all; this is a totally different gun and seems to be very well built. It will make a nice addition to my collection and am looking forward to taking it out, along with my Walther HK416D .22LR I picked up last fall...