I was going to build an all Colt M4gery and I ran into these uppers in the Bushmaster catalog. Has anyone had expereince with them?  For the same price I could have a Bushy Colt hybrid with the SIRS. Definitely have the cool factor.

Lot of complaints on the site Regarding Bushmaster lately.

I would resist the temptation to skimp, and go for the COLT.

You'll never hear anyone say that they regret buying a colt instead of another brand.

Quoted: I was going to build an all Colt M4gery and I ran into these uppers in the Bushmaster catalog. Has anyone had expereince with them?  For the same price I could have a Bushy Colt hybrid with the SIRS. Definitely have the cool factor.

Buy the 6400C.  It is a great rifle.

Colt for collector purposes and resale value will be worth more at any time than a hybrid. But I am not sure who makes the best gun overall to be honest. I have two preban Colts myself and a preban Bushy amongst others. And I have always prefered the bushy myself it seems to be of superior quality to me-an opionion only.
But buy or build what you want it is your money after all. One caution though about building one make sure you know what you are doing. While it is very "Chimple" to put most any of the AR-15 series together there are a few things you must pay very close attention to. Otherwise you may be on a website complaining about poor Q.C. at whomever.