I use it because I (mostly) like its interface, and it does get the job done. But it's a resource hog. I dread when it does a full system scan. Sometimes it will go on multiple "cookie hunts" in the same day. I'll see a second or a third one going on, and I think to myself, "You just did that! Enough already!" And the little reminder pop-ups in the bottom right-hand of my screen, ugh. Yes, I know that I have high disk usage at present. I can hear it. Duh?
I run ESET Smart Security, and it's excellent. It's the lightest weight anti-virus software that I've ever found. Please note, though, that the biggest danger probably isn't a traditional virus, but malware. I recommend running both ESET and something for malware protection, such as Malwarebytes Anti-Malware Pro.