I like Novell's slab menu system for GNOME, with its built-in search bar and most commonly used applications that bubble into the menu on demand; people used to Vista's search bar will probably pick up on it quickly. Also, instead of fly-out menus, the "More Applications" button leads you to a separate window that lets you browse or search for applications. (I hate having fly-out menus snap shut on me while I'm plowing through them, so this was a nice touch.)
On VirtualBox, the Guest Additions required installing some kernel header files from the repository, but worked fine otherwise. Unfortunately, openSUSE didn't work very well out of the box with either of the notebook computers. Display brightness controls on both machines didn't work properly, and for some reason the graphics detection routines couldn't pick up the native resolution of either machine's display. Both machines also didn't suspend and resume reliably, but they did come up from hibernation without problems. I also had trouble getting openSUSE to install on the Dell XPS, a problem due to the way grub chose to address the boot drive.
Finally, I also tried out the alpha 3 edition of openSUSE 11.0, although it locked up in the hardware detection phase on most of the systems I booted it on. I'll be looking at it again when there's a more stable build available.
For a lot of people -- mainly those just getting their feet wet with Linux -- Ubuntuis Linux. For Linux long-timers, Ubuntu's something of an upstart that's made good. Either way, any discussion of current desktop Linux distributions is impossible without mentioning Canonical's Ubuntu.
Rather than go with the long-standing 7.10 distribution, though, I booted up the new 8.04 edition -- which is technically still under development, but which ran well enough for me to talk confidently about it. It installed with little or no trouble on all of the test machines, including the VirtualBox instance, and within minutes I was firing up all the usual applications and getting right to work. I also nabbed quite a few updates (I used the geo-location service to find the fastest local mirror, which is neat) and had no trouble getting them in and running, either.
8.4 boasts a bundle of new features that manifested over time as I explored the system. One fairly major new change: consoles that normally require administrative access can be launched by a conventional user to inspect settings, but need to be unlocked (with the root password) to make any changes. Firefox 3 beta 4 is now included by default -- my experiences with it on both Linux and Windows have been really positive, and it looks like the final versions of both Firefox 3 and Ubuntu 8.4 may come at around the same time.
Power management in Linux seems to be perennially problematic if you use a closed binary driver, and Ubuntu was, unfortunately, no exception. The desktop machines suspended and hibernated fine as long as you weren't using proprietary video drivers, and the VAIO (with its Intel 915 controller) had a lightning-fast suspend/resume cycle. The Thinkpad also balked when using proprietary drivers.