I recently finished using the VMWare Update Manager to update my ESX Host systems. The process seems to work well but I did run into a few glitches. The first thing I noticed is that if it takes to long to migrate your VM's from the ESX Host you are updating to one of your other hosts the remediate process will time out. (The task can not be canceled and it ends up just hanging at that point). It would be nice if you had more information on what's going on with the task and cancellation points. The first time I ran the update I hit the task hang mentioned above. At this point I really only had two options restart the update manager service or restart the Virtual Center server. I first tried restarting the update manager service. In my case this did not work so I ended up restarting the Virtual Center server. After the reboot the tasks were showing as failed and no longer in progress.
So I decided to manually migrate all my VM's off the host before starting the process. On my first system I also manually put the host in maintenance mode. It took about 25 minutes to complete the process on each of my ESX hosts. One thing that was odd is that my versions didn't seem to change after the update process even though it showed 12 updates installed that were released on 3/12... VC still shows VMWare ESX Server 3.5.0, 64607. (this is what I showed before the updates.)
Overall this is still much easier then having to manually download each of the updates and run them from the console.