If had any compatibility issues between HMPA and KIS, I reported them straight away. But KIS started bugging with world of warcraft and sometimes with other software, which I had multiple times, and thus uninstalled KIS. However, I always returned... No specific threat I guess. Just the world at large The problem is always in these debates: you don't know what you will encounter or what will be created in the near future. Just to be sure that I won't loose any data, I already backup through the Carbonite service. And KIS feels a better performer than Defender if you go from all the reviews. But depending on the nature of the testing you're already covered for 85% and you don't know if you'll encounter malicious software in the bracket 85-99 that KIS covers additionally. Or that you're really lucky and encounter the 1% that KIS won't cover. Mix that in with the fact that the percentage of coverage moves on a daily (hourly?) basis and that AV suites themselves can be an additional attack vector and I find it quite difficult to really make a good assessment whether the extra cost, source of possible issues with regular tasks is really worth it.