The perceived security of IE7 (among those who are honestly trying it out) is the subject of a new TechRepublic discussion thread. Earlier this week I talked about IE7 security in relation to Windows Vista in this audiocast. I think IE7 is undoubtedly more secure by default. However, my concern is that it is so secure that it presents users with a lot of security messages and some users may get tired of seeing so many messages that they won't pay attention to them and will figure out how to automatically click through them, or worse, will figure out how to lower their security settings so they won't get as many annoying security prompts. Still, IE7 in Vista will make it much harder for malicious software to get a foothold in the OS because the browser is well quarantined within the OS.
If you'd tried IE7, I'd like to hear what you think about it.