From what I can gather, the definitions that are saved onto your computer for checking and matching for spyware is in a poorly secured file (making it easily editable by those who want to change it). And the program appears to be written generally quite badly.
I had Ad-Aware but i found that it just get's in the way really. And it's slugish, i agree it's poorly writen but Spybot SD is good and all but it tends to eat my resources up