SimplyMEPIS has been one of my favorite Linux distributions for quite some time. They focus on simplicity, stability and usability, and as such they were one of the last distributions still using KDE 3.5, in their 8.0 release. I was pleased a few months ago when they started development of a new release using KDE 4, and I was very impressed with how stable it was even in the very early pre-releases. That development and testing cycle has now culminated with the SimplyMEPIS 8.5 Release Announcement. It is interesting to note, however, that in keeping with their conservative approach and support for existing users, the 8.0 release is still available.
MEPIS is based on Debian GNU/Linux, so if you are looking for a good alternative to Ubuntu and its derivatives, SimplyMEPIS is definitely worth a try. With so much controversy over the changes which seem likely to be made in the upcoming Ubuntu 10.04 release, I suspect that MEPIS might pick up quite a few new users. (In fact, MEPIS tried switching to Ubuntu as their base a while back, and subsequently decided to go back to Debian.)
With this release, MEPIS updates the Linux kernel (to 2.6.32), the KDE desktop (to 4.3.4), and pretty much all of the major and minor packages as well. That gets you all of the latest device support and usability developments, but it still looks and feels very similar to their 8.0 and previous releases, so existing users won't feel like they have been "pushed into the deep end" all at once.
I personally consider SimplyMEPIS to be "Debian plus all the changes and good stuff that I would probably choose to make myself". In this regard I am intrigued by a comment made in the 8.5 release notes, which says that although this release is in LiveCD-sized images, there may be another release in the near future, of a larger size with even more included packages. That would be a very good thing!
I'm pleased to see this release get out in front of the coming wave from the other major distributions.