For melee attacks, it's fine. For ranged attacks, it's weird, but I can get over that.
The real issue is whether standing up should still be a Free action. In Infinity and Conan, Stand is a Restriction (or Minor) action. I'm considering that "errata," which helps Knockdown be somewhat more effective (basically you can't move very far if you get knocked down and want to stand up).
I was also thinking about adding Vicious 1 to Knockdown, but then I worry that (1) nobody will ever use a handgun that does NOT have Knockdown, and (2) there will be too much Viciousness.