Results vary, but ....
Honestly, people tend to get a radically different perspective on Windows vs. OS X in a corporate setting than on their own machines at home.
Mac OS X has been focused squarely on the needs of a home, small office, or even educational user - with "enterprise" and "corporate" needs more of an afterthought. They're starting to address some of this now. (Leopard improves on a number of basic network file/folder sharing issues in OS X that Windows had them beat on, historically.)
The virus and spyware issue is VERY real, and the *only* reason you don't really observe it much in a corporate setting is because businesses make use of good firewalls, have policies implemented that discourage a lot of "personal/entertainment" uses of employee computers, and typically keep anti-virus software subscriptions up-to-date at all times.
I do a lot of on-site computer service for a living, and I quite regularly run into Windows XP based machines, fully patched and updated with "Service Pack 2" and the other Windows "security updates" Microsoft offers via automatic update. Yet, they're so gummed up with spyware and trojan horse virus programs, they can barely boot up or function. All too often, you have to reformat the whole hard drive and reinstall everything from scratch or good backups to eradicate the problems.
I ran OS X Tiger on 3 different Macs at home myself, and have 3 different networked printers on my LAN as well. I never really had any printing issues, BUT, I've observed it on other people's setups. Just like Vista, it usually boils down to poor drivers. Especially in the case of "all in one" scanner/fax/copier/printers, drivers are flaky in OS X. They may work fine on a local printer attached via USB cable - but as soon as you try to attach it to a printer sharing box, giving it an IP address, everything falls apart.
I've found that with printers, you "get what you pay for", in that inexpensive printers usually only work well when directly connected to ONE Mac or PC. If you want to print to one over a network in any way, shape or form, you need to shell out the money for one that's clearly designed for heavier "business" use. HP, for example, just won't put the effort into making their drivers reliable over a network on the "under $200" inkjet printers....
Sadly, we've had more stability issues with Tiger than Vista within our organization. While the problems in vista seem to be more driver availability and some incompatabilities with older software, with Tiger, we've had all kinds of weird issues with finder content not refreshing, printing not working at all (to any printer) until reboot..etc. Just weird things that seem silly in this day and age.
We even had two machines side by side looking at the computers on the network and one would see machine called "X" lets say and the other would not show it in the list. No matter what we did it wouldnt show. Even upon reboot it failed to show. Very weird. Both were running 10.4.x at the time. It was a 2K3 server that failed to show but other 2k3 servers we had still were there.
Anyway from my experience in our environment where we have 100+ XP stations, 10-15 Vista stations and 20 OSX stations (10.4) I can honestly say that Apple is grasping at straws with the ads. You may not like the UI but all the talk about stability and viruses and all that being a major issue is a farse. In the 4 years of being at the company, I have yet to see 1 comprimised computer (Virus, Trojan or Malware). All the ads are doing is annoying the people who are computer literate enough to use both platforms and the ones that fine them believable are the inexperienced and gullable. Thats my take on things. Even speaking to the hard core designers who have used both platforms, they find the ads unprofessional.
If you've ever seen a really small cocky guy being all loud mouth and pushy even towards the big guy... Well you know where I might be going with this. I believe in speak softly and carry a big stick. The ads IMO show apple as the 'little' guy who makes a lot of noise. One day he's gonna get hit. LOL.
Really, with MS office being such an important software for a lot of people, all MS would have to do is stop development of that product on the apple platform and it would do major damage. Not sure if anyone else feels this way but I think its retarded to poke the sleeping bull with a big stick....