You really do need a "belt & suspenders" approach to spyware. No single product is even close to being sufficient on it's own. Most installations that I support use Symantec AntiVirus throughout. Starting with version 10, they've included some rudimentary spyware protection. Actually the protection really isn't all that bad, the management of it is. To supplement this I have repacked Spybot S&D and that gets installed on everything as well. Just for good measure I pump out all of the registry entries generated by SpywareBlaster as they are updated. For those machines that are dedicated to a single user, usually a single user who "needs" administrative privledges (think bosses), I also install Microsoft AntiSpyware.
I don't buy the whole Opera / Firefox argument either. For starters, only a masochist would try to deploy and manage that in the enterprise, not to mention the cascade of help desk calls from deer in the headlights users who would be scared to death of something they've never seen before. Second, a good admin would use WUS, SMS, or some other solution to keep every workstation up on security patches. Third, IE is manageable through AD. The last point is the one that matters most to me. I've gotten to be downright draconian in my management of IE, yet everyone is still able to get their work done, everyone can still run their little eBay business from their cubicle, and in the last 8 years I've had ONE problem with spyware. And that was.... c'mon guess....

on the bosses computer. The guy who had to have admin priveledges. The guy that gets paid six figures to surf TwistedHumor.com and some other unmentionables all day long.
Sorry for all the sarcasm. I had to suffer fools all day this happy Monday.