Well for people who scan everything they install I'm sure that's a handy feature. I don't scan things I install, ever...and have had 0 viruses on my laptop to date. Actually to mention it, I don't think I've ever had a virus that I can remember, on any computer I've owned. It helps to not use questionable software and to be on a secure network at all times. If you're on the go or on an un-familiar network/connection, don't start mass downloading freeware programs...or just don't connect.
But let's simply look at the basics here. Norton is resource heavy on your system, therefore it slows it down whether you can "tell" or not. Actually if you want to go even deeper, just about anything from Symantec is what we like to call "bloated". Does it do a good job? Well that's for the consumer to decide, different people need different things.
Simply Google "Norton System Hog" and you'll get thousands of results and people who have to agree. The bottom line is it sells pretty well do to clever marketing, but under the professional appearance it's a pretty bloated bundle of softwares and can be as effective (or more accurately, as un-effective) as many FREEWARE/SHAREWARE products.
I'm bored so let's all take a look;
http://reviews.cnet.com/Norton_AntiVirus_2006/4505-3667_7-31473733.html
http://sweetpoison.wordpress.com/2006/02/23/symantec-admits-using-rootkits/
Well I won't post them all here, this is just turning into a huge bash. ANY AntiVirus software is going to have holes/security flaws because it's a never-ending battle. But Norton just seems to always be on the ugly side of things. Just looking at the vast majority of people here who hate it, and doing a few Google searches...you can get the picture that's it's not all it's cracked up to be. I used it for several years no problems...it never RUINED my computer. But it was definetly a system hog, I had my fair share of update problems and having to remove it and re-install it several times, etc. You may get the same kind of protection/performance/problems out of it as you would with say "AVG"....BUT...something like "AVG" is FREE and I like free.