The most recent PC World article Bigger Threats, Better Defense posted the results of tests performed on a various group of AV scanners. If anything, it appears (according to this article) that Norton actually has a better detection rate then most of the AV's tested. There were some drawbacks as well (slow scans, system hog) as mentioned in this thread already. Amazingly enough, AVG Pro didn't do very well posting the worst detection performance of the AV's tested. Of course this is only one test, with certain pre-established criteria that's hopefully unbiased and not influenced by any ' hidden agendas'.
It takes more than one crappy review for me to change my mind about a product anyways.

As for running multiple AV's at once, I recently found that to be a problem on my system. I had Norton 2003 and more recently, AVG 6.0 (free version) and there apparently was some kind of conflict with AVG's startup file Avgcc32.exe and Norton's startup file(s) ccEvtMgr.exe and/or ccRegVfy.exe that would cause my system to halt upon bootup. The crashes were rather consistent until I disabled AVG on bootup and started using it, as Nihil said, an on-demand application. I've had no problems since then.

After I finished this post, I went about checking for what actually sets the standard for AV testing criteria. Anyways, if you've been wondering what the hell the Wildlist is that's always being thrown around in articles, well you can see the most updated Wildlist Here
I thought it might be interesting to know.