I haven't used this much, since I have the Windows version of Safari for testing the rendering of WebKit as an engine and it has at least one show-stopping problem for me: scrollwheel behaviour is reversed and there's no option to change this configuration. Not that I use Safari much either, since that doesn't respect my system settings for scrolling either...
You'd expect browsers with minority share not to hobble adoption by not respecting system settings, wouldn't you? But I digress.
The fact it turns up with a Vista theme and no way to change that isn't too appealing either, but the general design approach of conserving screen space is welcome.
Also welcome, in a "this is mindbogglingly simple and useful to many people, why haven't developers thought of it before?" way is the built-in Incognito Mode. Create a new browser window and any pages/cookies rendered in it won't be added to your browsing history. IE8 is slated to have this when it's released, and I fully expect Firefox (and Opera, if it doesn't already) to gain an Incognito Mode very soon. You can create a new profile in Firefox with custom settings for much the same experience, but Google have stolen a march by having the option ready-to-use.
Naturally this is going to find most application for people browsing porn, but it should also stop a lot of people getting caught out when buying gifts for relatives, searching for potentially embarassing medical information, etc. — it's inconvenient to wipe out your entire browsing history or pick through a list manually deleting some entries but not others, so people don't usually do it (even if they know how.)
Adding convenience is always a winner. I reckon Incognito Mode will be particularly useful in Firefox, combined with the default behaviour of the 'Awesomebar' — which is to search your bookmarks and history for any keywords typed into the address bar.
Along with the scrolling, also annoying is that, for some reason, Chrome installs to C:\Documents and Settings\[user]\Local Settings\Application Data\Google\Chrome\Application rather than to Program Files. (I was looking for the executable to see if adding an exception to KatMouse and letting Windows handle scrolling for that program would enable normal scrollwheel behaviour. It doesn't.)
Most irritating feature saved until last: setup buries the auto-running of a Google Update process, which is launched on system startup. This kind of scummy practice I expect from Apple (eg, iTunes loads a service for handling iPods whether you want it or not) or RealPlayer-like junk.
But still, an interesting few minutes and something that bears watching.