Before I did that I forced Time Machine to do a "back up now" to make sure I had a current picture of my Mac. Once the download was complete it spent some more time doing installs (about 5 minutes or so), then prompted me that it needed to reboot the machine.
The reboot process ran about 5 minutes (spinning gear) before it prompted me to log in. Once that was done I was up and running, though not everything was available immediately. The Dashboard was not available initially and a quick look at iStat menu showed some pretty heavily sustained CPU and disk activity. Dashboard came back after a little while but Spotlight was still out until 12 minutes in because it was "indexing the backup".
I have a 1TB MyBook drive attached via USB that I run TimeMachine against. Since I have a 320GB HD in the MacBook that means a fair amount of data to reindex.
Once Spotlight became available the CPU went on another work bender with MDS and MDWORKER cranking pretty hard. These are used by Spotlight for reindexing and made the machine a bit sluggish while they were working. It was still useable mind you, just really busy.
A full 25 minutes after my reboot the machine was still cranking away with indexing tasks; both MDS and MDWORKER using a sustained 20% CPU. I'm watching my MacBook as I write this on my MacPro and at 38 minutes in the machine is still going through peaks and valleys of CPU usage.
While it's probably safe to shut down the machine or put it to sleep while the MDS and MDWORKER process is happily working away I wouldn't recommend it. Best bet is to perform this upgrade when you can let the machine sit and bake for a while - plan on an hour - then letting it complete everything.
If you have any feedback on the 10.5.3 release that would be of help to others please add it in the comments below!
Update: Over an hour later it's still busy indexing away. Note the iStat results:
Final Update: It took a little over 2 hours for the machine to finally settle down and complete the reindexing process.
Really Final Update: It appears that this was related to having an attached USB based WD MyBook drive (1TB), which I use for Time Machine backups. When I upgraded my Mac Pro the indexing after the final reboot lasted a couple of minutes with very low CPU impact.