I too have only used Microsoft Security Essentials/Defender for years without problem. I run all new/experimental/potentially harmful programs/websites in a virtual machine first before running it on my host machine.
I run all new/experimental/potentially harmful programs/websites in a virtual machine first before running it on my host machine.
Admittedly, I'll have to apologise for referring to Windows Defender while thinking of Microsoft Security Essentials (although you'd think they were the same product).
The reports I often look at can be found here ... Dennis Technology Labs
Personally, I'm still unlikely to install any additional security products. But, when in doubt, additional security is definitely welcome!
They actually are the 'same' now. "In Windows 8, Windows Defender replaces Microsoft Security Essentials.": Windows 8 | Security Features
That's great, but I'm looking at this from the fact that the OP asked the question in the first place. Unless he supplies more background information, I'm inferring that (1) he's not a power user, and/or (2) he may not have another full Windows machine. Power users by default already know the answer to this question, ergo we can get by without protection if we take the necessary precautions and know what we're doing. Simply by asking the question, I think he should. The only worry I'd have is not performance related but how that might impact battery life; perhaps some products have a mobile mode.