I reckon it cuts down loading times by at least 30% from FF3, if not nearer 50%. Much quicker at loading things like PDFs, things that use Flash, etc.
It also seems to complete a page quicker, if that makes sense. So, if I go to open a page, once that page shows, it loads instantly, there's no delay for it to download some other data from that page.
FF3 did need an update as more flaws were creeping through with other browsers doing well (Chrome being quick, such an unfriendly interface imo).
I don't see too many new features. A little button to open a tab, but don't see point in it, Ctrl+T does the job. 3.5 is all about speed for me. I expect all the security I get with FF. Not too many interface changes - looking at their release notes - but quite a lot of background work, which is why it's so much quicker.
Fans of FF will get it and will be happy with it. I've not tested the other ones for a while (not going to be both with even trying IE8, it gets used - forced - for doing Windows Updates, I could load 10 tabs on FF3.5 in the time it takes to load that one page.
This is turning in to a review!
Despite the lack of new interface features (that I can see, might be missing) I don't feel it's missing anything, I don't say, "I wish it had that.".
To summarise: Get it. It's quick.