Grant has already directed you to my site, so I won't duplicate any of the information found there.
Regarding Intego, its detection rates are quite good (see my recent Mac anti-virus testing). However, it does tend to be a bit "heavy," installing all kinds of things that can disrupt normal operation of the system. Multiple layers of security are a good thing... you will need to decide whether you actually need the things that Intego offers and whether they are worth the cost you will pay (namely, performance, possible network issues and possible stability issues).
I have to quibble about one thing...
Multiple layers of security are a good thing...
Multiple Anti-Virus programs are an exception to your statement.
Installing more than one Virus program that "runs in your system" will cripple any computer, because everything it does "looks like a Virus" to the other program.
multiple backup sets, methods, strategies.
Router, security in the OS, in web browsers.
I doubt any intention to even suggest using duplicate programs that do the same thing.
Having a system backup clone from last month, from before the security update Apple issued, indispensible. And part of any system strategy to protect your files and data.
And always coupled with good old common sense: don't touch that download link for Flash while on a triple-X site, or "our site needs a special plugin to display content" rouse.
Yes, you're right, when I talk about multiple layers of security, I'm definitely NOT talking about multiple anti-virus programs installed at one time! As you rightfully point out, multiple anti-virus programs can be a very serious problem. Sorry for that confusion!
By "multiple layers of security," I'm referring to using various technologies to protect your machine in more than one way. Some examples include using Gatekeeper effectively, disabling Java, using some form of "click-to-play" for any browser plugins, having some kind of internet filtering (even if that's just a browser that supports Google Safe Browsing, like Safari), etc. Anti-virus software is only one layer among many, and it's not the strongest layer by far.