Some people here look down on anything that left the factory less than a complete weapon. Others are adherents to one brand or the other (or two or three) to the exclusion of others.
I have mixed and matched various brands for years to come up with exactly what I want for each rifle, and I have been well-served by this approach. Stick with quality components and you shouldn't have problems. Bushmaster and DPMS are both good brands - if you like the layout, value, etc. of the DPMS upper, then go for it! Good luck!
edited to add:
The above having been said, I usually get around to putting a lower of the same brand with any given upper eventually, then getting a new upper for the newly "abandoned" lower - but I buy, sell, and trade ARs more than most. I eventually match up brands because of some personal O/C issues - not because of compatibility problems or anything like that.