Updating OpenSolaris should be straightforward. The documentation says that you need just to run following command:
$ pfexec pkg image-update
This way you'll get updated to latest bi-weekly bits (currently at build 97). If updates worked as advertised, it would be perfect, however I got burned several times during updates, so I would like to warn you before you waste your time with unsuccessful updates.
At first, read the release notes. Pay very special attention to the IPS instructions. These commands should be executed before you install any packages or perform updates:
$ pfexec pkg refresh
$ pfexec pkg install SUNWipkg@0.5.11-0.86
$ pfexec pkg install firstname.lastname@example.org
$ pfexec pkg install SUNWipkg-gui
Why is this important? The version of IPS distributed with 2008.05 had several major bugs. I don't want to go much into details about these bugs, all you need to know is that they are really ugly and can even cause your system to become unbootable. So run these updates.
Second, before running the pkg image-update command read the update notes. Again, these are very important and if you try to run the update directly from 2008.05 install your system may become unbootable (luckily the old boot environment will still exist but you won't be able to boot into the new boot environment, so you waste a lot of time - these updates are not exactly small).
It is NOT possible to update your system using the Package Manager GUI so you need to use the command line to do the big update.
Finally, a word of warning. I updated my system several times. Many other bloggers reported successful updates to latest builds. I guess they don't use that much of the software, because there have been many issues in recent builds - e.g. the package manager wouldn't start, Firefox wouldn't start, linker wouldn't link, etc. At the end I decided to stick to build 86 (the original OpenSolaris 2008.05 install) because I couldn't get my system ready for a presentation - there was always something not working.
Perhaps all these issues have been resolved in recent builds - I certainly hope so (and yes, I did report some bugs related to updates). If you are just a regular user of OpenSolaris and you do not want to spend your time with troubleshooting some of the issues that may happen after update, my suggestion to you is NOT to update. Wait for the next release (coming soon - in November). On the other hand if you want to help the OpenSolaris project and have some time available to test latest builds and report bugs, you'll want to run the latest bits.
We certainly want as many people as possible trying the latest updates, but from what I saw in other blogs it looked like everyone should update to latest bits. I recommend to think twice before doing so, you might get burned. I think upgrading just to the next major release is fine for regular users of OpenSolaris and even though running "pkg image-update" looks simple enough things can go wrong. You have been warned :)