My team used a mix of UMP40s & G36KE2s from 02-14. When I arrived in 12, I was assigned a G36, which I carried for two years until we converted to Colt 6946s. My observations are as follows. Apologies in advance for a lengthy post.
Upon being issued my G36, I thought it was the coolest looking rifle ever but this quickly wore off with daily use.
The mags were much bulkier than an M4 mag, which meant only G36 specific mag pouches worked. The only options were snap and velcro (we converted to MOLLE in 2011 but I assume protech wasn't making G36 pouches at the time) or German surplus in flecktarn (we wore black kit with navy blue uniforms at the time, so that wasn't flying.) Thankfully literally weeks after being issued a G36, Magpul released the 30G, which solved the issue as the personally purchased 30G fit in most M4 mag pouches (also personally purchased). I looked into the STANAG magwells, but they were expensive and the HKs were slated for replacement as soon as the money was approved.
The feedlips were fragile and we had frequent breakages from dropped mags during training. Again, the release of the 30G solved this issue.
If you clipped two mags together using the locking pegs, it really killed the handling of the rifle. Additionally the top round of the spare mag tended to walk out a bit under recoil. which created obvious issues when you went to reload. For this reason, team SOP was that G36 mags would not be clipped together.
The manual of arms was very inefficient compared to an M4. The typical HK selector placement could not match the ergonomics of an M4. Paddle mag release was also far, far slower than an M4s button release. The ambi toggle for the bolt underneath the optic rail was the real issue that I saw as you had to keep your hand perfectly flat to reach in and catch it. I knew our HKs would be replaced relatively soon with M4s, otherwise I would have invested in the KAC irons / rail. Again, none of this made the rifle unusable but when I was attending carbine courses and shooting against students with M4s, I became keenly aware of the ergonomic shortcomings.
LOP on the stock was too long and non-adjustable. I purchased a C length stock from HKparts and this helped, but it was still fixed length. Again, if they weren't slated to be replaced I would have considered personally purchasing an IDZ or KV stock.The folding stock was good for when we did vehicle stuff. I thought it was a nice design feature to have holes in the stock for retaining the takedown pins. I added skate tape to the buttpad to give it some extra purchase.
Before my time, but the rifles were originally purchased with the integral 3x/1x optics. They were pretty unpopular, so they were replaced with 1913 rails and Aimpoint M2s. The "iron" sights on the rail were pretty much useless, I don't recall cowitness being possible. I replaced my M2 with a personally purchased T1.
Because of their age (AC date code) the handguards did not have a heatshield. They were just plastic. The handguards would heat up and become uncomfortable during high round count days. They also had no accessory rails, except for the HK UTLs, which by that time had all broken. I was advised to tape a 6P to my handguards with 100 mph tape. I personally purchased a then current production handguards from HKparts which had rails at the 3, 6, and 9, as well as a heatshield. Mounted a magpul VFG and personal purchase surefire m600 with larue QD mount.
Sling were either HK 3 points or old school bungee cord single points - enough said. We attached them using expended flash bang split rings looped through the snap hook mount at the rear of the receiver. I used a magpul MS3, and mounted an RSA on the handguard so I could switch between single point and two point.
Due to the significant use of plastics, it was a very bulky rifle. All of the weight was in the BCG and the barrel so it was front heavy and wanted to "pull" forward and down in your grip - we added skate tape to the pistol grip to help with this. Despite having a 12.5" barrel it was the same length as a 16" M4.
Trigger group was all plastic, with some stamped metal reinforcements. It wasn't anything to write home about, pretty mushy with some creep and a blah reset. A GI trigger in an M4 puts it to shame.
Recoil impulse was more / different than an M4. In semi auto it wasn't that bad, but doing full auto strings (required for qual) it was noticeable. Again, I probably wouldn't have noticed as much if I wasn't attending carbine courses full of guys shooting M4s.
Very easy to disassemble. Very easy to clean - the bolt had the typical HK space magic finish so it wiped clean with a rag.
I recall accuracy being pretty decent. I don't want to sell the G36 short, as I wasn't that great of a shooter at the time, and I had no real metric against which to compare the G36. It always seemed to do fine for us and we never seems to experience the wandering zero issue that others had. But again, I was a new guy and wasn't keenly aware of such things.
The G36 was supremely reliable. It just ran and ran and ran. No parts breakages that I can recall either. That's in stark contrast to the guys who were stuck with the UMP40s, which seemed to constantly suffer issues at that point in their service life - about a decade.
Whoever said the G36 was the product of a post Cold War / post reunification Germany, where they needed something that was cheap and worked, is dead on. All in all i felt the G36 was a pretty decent rifle, and if HK ever comes out with a civilian G36, I would buy one for nostalgias sake. But it seems everyone else has come to the same conclusion - it is just so hard to beat an M4.