I'll put this disclaimer up before I make any comments...
I've been around computers since DOS. At one time, being a dedicated Netscape user, I have over the years developed a deep seeded resentment of all things Microsoft. Sorry.
So, understanding that my comments are a bit tainted, now to the question posed in the title to this thread...
Though IE 7 looks interesting, as a typical MS product, it's a direct conduit to the heart of the operating system, and people will lick their chops figuring out ways to exploit it.
In the long run, Vista will probably end up being a bigger mess than XP is now. As they say, "Where there's a will, there's an angle."
I have tried Opera, and I like it a lot. But, I use Firefox primarily because it's open source. (IE on my machine is a dedicated slave to MS update.) In my opinion, either Opera or Firefox, would certainly be better alternatives to IE.
Your choice as to which one to use.