I don't think it is, at least until I find a useful alternative to NoScript/NotScript that works with IE. Granted, that isn't vanilla browser, but rogue scripts and malicious ads are a very common attack vector, and AFAIK IE lacks any good method of dealing with them.
It is, though. The implication that using IE9 is going to cause your computer to become "infested by viruses" by virtue of choosing IE9 as your web browser is ridiculous. The type of browsing behavior that would lead to such an extreme situation would be less than safe using most mainstream browsers. By most accounts IE9 is at least as secure as Firefox or Chrome. Yes, NoScript is awesome, but what percentage of the Firefox installed-base uses it? Anyway, I use Firefox and I'm in no way advocating IE, but misinformation drives me nuts.
I mostly wanted to address the speed comment because IE9 actually really surprised me in that area. When I run automated client testing against our web sites and web services at work IE9 completes significantly faster than Firefox does.