It's certainly a factor I consider in any firearms purchase.
Based on the type of firearm and its intended use, it may not be a major factor. I do own a single-shot rifle, for example. I also own some 6 shot revolvers as well as some other handguns and rifles which would not be affected by a 10 shot magazine size limit.
That said, for a home-defense firearm, particularly a handgun, capacity is certainly an important consideration, in my opinion.
Handguns are easy to miss with, and do not have nearly the terminal effect that long guns do. Couple that with the possibilty of more than one attacker, and a lot of rounds on tap starts to look very attractive.