I've bought it and use it on my main PC. No slowdowns, barely notice it's even there. I'm just happy in the knowledge it's protecting me.
Also the Devs are very quick to respond to any bugs/incompatbilities and releasing beta's. Be warned though if you use software with similar functionality (EMET, MBAE, BitDefender's ATC) you are more likely to get problems.
If you are a gamer, then you might want to look elsewhere as the Anti-Piracy methods used by games can trigger HMPA because they use similar techniques to malware.