Just installed 10.5.6. No problems, following the usual apple guide to good
housekeeping:
1) close all programs
2) REPAIR disk permissions (irrespective of whether you ran it the day before).
3) Download and install software
3a) On MBP, or Macbooks, ensure your AC adaptor is connected (irrespective of whether battery is 100% or not).
4) Enter password (if asked) and install.
My Mac Pro took about 5 minutes downloading the 190mB (probably Apple is busy right now) and about 1 1/2 minutes to install.