As most of you will know, Scientific Linux is a free clone of RHEL compiled from the original source rpm's, and with the upstream branding removed. As such it is almost identical to the Red Hat product, but in contrast to the CentOS project the SL team are adding and tweaking a few packages to make it better suit their needs.
It is cool to know that the people responsible behind the Large Hadron Collider are putting this together, and it makes me feel that on top of the proven reliability of the enterprise grade Red Hat product there is another layer of hugely competent folk that cross check and add their own finishing touches. As this distribution is used across many scientific sites and labs it has to have a sane base, be usable on laptops, and easily customizable for different sites and different spins. I imagine the labs will have somewhat different requirements from laptop users and admin staff. Scientific Linux has added wireless firmware and tools and a few packages that make life easier to the official Red Hat, and that's a point in its favor for the user who would like to take advantage of the power of an enterprise product.
Another plus is that in addition to providing their own documentation and repositories those resources available for RHEL and for CentOS also apply and can be used, like the EPEL repository which provides more up to date packages backported from Fedora, and of course rpmfusion. Or the RHEL release notes and documentation.
In search of a good fit I recently installed SL 5.4 on the new Acer Aspire 5551 together with a few more distributions, and, updated to 5.5, it quickly became one of my two favorite distributions on this machine. What surprised me was that something that has been released nearly three years ago supported this relatively new hardware without any problems whatsoever and runs immensely stable. This is due to the kernel still being updated with every point release, adding many backported fixes and support for new hardware through patches in the upsteam kernel. For your desktop use OpenOffice.org 3.1.1 and Firefox 3.6.11 are available, so it is really quite up to date.
Nevertheless, when I found out Scientfic have a testing release of 6.0 out I had to download and test drive it. It is currently in the 6rolling directory and is available like you would expect for both the i386 and x86_64 architectures. There is no documentation yet to be found, but here's a link to the official Red Hat 6 installation guide and other Red Hat 6 documentation. The ISOs are dated 15/10/2010 and are around 1.79GB for the 32 and 2.0GB for the 64-bit version. That's about a month old by now, but still, not too outdated to check it out.
The installation went smooth and very much like any install with Anaconda, the Red Hat Linux installer, and basically identical to what you found in any recent version of Fedora.
The guys behind Scientific Linux have stayed with a theme similar to upstream in the sense that it is of a darker blue color tone like it and has the same abstract feel. It will make sense if you compare screenshots of the two.
Using Gnome, several of the panel applets crashed almost immediately and remained unusable, notably the trash applet, the clock and calendar and the network applets. Firefox would not load at all and so wouldn't the release notes, but at this time this is probably a dead link. That's fine, it's still early days for an alpha. Apart from that it performed admirably fast even using the default Gnome within a virtual environment and reminded me in desktop responsiveness of one of the lighter distributions I often use.
The major version bump has of course introduced more changes than I could mention or understand, so here's a link to a technical look at Red Hat 6 and introductory videos. On a side note, Red Hat have also introduced a new certification scheme and the Red Hat Certified Systems Administrator (RHCSA). One of the probably more interesting ones for desktop users is the refinement of virtualization technology and the switch from Xen to KVM (but also already introduced to the 5.x series with update 4). The ext4 file system is now included as is btrfs support. Ext4 is interesting for general users from the perspective that it does much faster consistency checking on larger volumes. Version 6 now also has the ability to merge LVM snapshots, if you are using that that is and not normal partitions.
The rest is pretty predictable software updates, like Gnome 2.28 instead of 2.16 and KDE 4.3 instead of 3.4, and the rest of it. Sure, many people will find it of great importance to have a stable and solid distribution with refreshed software, but like with Debian stable it is already a bit outdated. I for my part am happy with Gnome 2.16, after all for playing multimedia files for example it's the codecs that matter. Totem 2.16 will play my movies just as well as Totem 2.28, and many other current desktop apps are provided in Scientific Linux and through other repositories. I have not been a fan of Gnome after 2.16 with all the changes going in, like the icons disappearing from the System menu which gives the whole desktop an inconsistent look. I also missed the icon for the open terminal extension in the shell menu that has been in Scientific Linux for a long time and which I consider very helpful. The icon used to mark it out, whereas now I find myself searching among the entries every time. With 5.5 even connecting my wireless I'm not in a hurry to switch.
Many people thought RHEL/SL 6 will be based on Fedora 12, and on the surface it pretty much feels like that. If you were happy with recent Fedora releases and want more of the same but rock solid and with long term support, then once Scientific Linux 6 final is out this should be for you.
There's no real reason though to upgrade, unless you desperately need some of the newer packages for development, or you want a newer desktop interface that is nevertheless already a year behind and in my opinion (be it KDE or Gnome) in some ways worse than its predecessors in SL 5.