I have a Carbon 15 Type 97S, which has a full length picatinny rail on top (from the rear sight to the end of the handguard). This model also has integral sights. I intend to mount an Aimpoint when I want to lay out the cash.
What I like about this rifle is that it already has quite a few of the modifications that I would have had to do on my own (full length rail, Hogue grip, BUIS) if I had an M-4. I really like the light weight, which makes handling extremely fast, not to mention carrying is as effortless as possible. I haven't had any functioning problems, either.
If I were going to improve anything on this rifle, I would have included an ejection port cover. Additionally, the air-hose linkage, which connects the compensator to the muzzle, can disconnect when I release the bolt with the muzzle pointed down. This has only happend on an empty chamber, though, never when firing. I would also like the ability to mount a foregrip.
Overall, I'm really pleased with this rifle, and I hope that this innovative product succeeds, under the Bushmaster name, where Professional Ordnance failed.