I have a Holosun HE503GU-Green on my Micro Scorpion, a Holosun HS503GU-Red on standard braced Scorpion pistol and a Holosun HS503GU-Red on my FA HK MP5. I have all 3 setup for the circle dot reticle, which works perfectly for me. Anything inside the 65moa circle at 25yds is very quickly a solid hit. The 2moa dot is perfect for more precision (hostage taker head shots in shoot house) and longer ranges 100+ yds. The Scorpions are in the low mounts directly to the rail and they co-witness perfectly with standard sights for me. The HK uses the lowest profile mount I could get and with the Holosun using it's low mount, it also co-witnesses with the factory sights. So far, (knock on wood), all have worked perfectly. All 3 use the standard CR2032 battery in the side mount screw on cap (now discontinued), which makes it extremely easy to change out batteries when needed. All 3 models have the Shake Awake feature to conserve battery life.
I have a newer model Holosun HS503CU that utilizes a side mounted battery tray that uses incredibly small headed torx screws to secure the battery tray (you have to utilize their driver tool of which I have already stripped the bit on the one provided and they did send a replacement) and it is a huge pain in the ass to change the batteries. Whatever bean counter or engineer that came up with this battery tray design needs to stop sucking oxygen permanently.