Although it was a nice idea, the original Professional Ordnance Carbon 15 used proprietary parts which if any break, you have a nice paperweight. No longer available even from Bushmaster.
If you are talking about the Bushmaster Carbon 15 which is the Pro-Ord rifle with updates to allow it to use standard parts, they are not bad light weight AR variants.
Personally with all the standard AR-15s around, I would stick with them.