My organisation runs about 500 PCs all using IE6. Half the PCs are available to the general public and are locked down by software on the PCs, Group policies and the network (proxying, ACLs etc.
All of the PCs run antivirus software and we have a regular patch program. In the near future we intend to set up an update server.
We have had no problems with IE since we had IE5 years ago. We recently DID get a trojan but it had nothing to do with IE. It got in through an OS exploit and our AV provider didn't update in time to save us.
Seems to me IE is secure enough if you put some effort in. And besides, I have no confidence that any software is invulnerable to hacking. The number of IE installations makes it a big target but also ensure that the world quickly becomes aware of problems, and MS reacts pretty well. Maybe some of the software with smaller installed bases are safer for now, but when they are hacked, how quickly will the cavalry arrive?
I think there is a big problem in the IT world with anti-MS sentiment being paraded as good technical sense. I remain to be convinced.