I upgraded to El Capitan and McAfee Internet Security quit working. None of the info in this post worked for me. The mcafee uninstaller would not run on El Capitan and the newly downloaded installer from AT&T/McAfee was failing to install and unable to remove the old version.
Not having AV on a Mac is probably not a good idea. I have had McAfee Internet Security on my Mac for about 2 years now with minimal system impact and I do a lot of photoshop, Xcode, Parallels with multiple VMs, etc. (granted I do have a powerful macbook pro 16GB 11,3).
With the growing popularity of Mac over the past several years OSX has become more of a target for opportunistic cyber criminals.
So all OS's have vulnerabilities from time to time and tools such as antivirus and intrusion detection/prevention can reduce your chance of finding yourself with a compromised Mac.
What worked for me was to delete all of the references to McAfee in /Library/Application\ Support/Mcafee, /Library/LaunchAgents/..., /Library/LaunchDaemons/..., and finally delete the McAfee app code lingering on my mac with: sudo rm -fr /usr/local/McAfee/
After deleting all traces of the old non-El Capitan compliant McAfee, I was able to successfully install the proper McAfee Internet Security that works with El Capitan.
WARNING: Make sure you know what you are doing before you try to force remove a folder with sudo rm -fr.
Q: Does AV and other tools impact mac performance. A: Yes.
Q: Does it impact my ability to use my Mac for many different purposes. A: No.
Q: Does it reduce my risk of getting my mac compromised. A: Yes. How much does it reduce my risk? Hard to say. Depends on signatures from the AV provider and how I configure the firewall. How good the intrusion prevention rules/signatures are.
Q: Would I be better of if I chose not to use some form of AV, firewall, and IPS. A: Probably not.