Buy it now.
Conventional wisdom says never buy the first release of any Apple OS upgrade unless you want the bugs that were missed in Apple's rush-to-market. Software Smarties will say wait for the first revision of the upgrade, such as Tiger 10.4.2 .
Of-course by then we'll have news that OSX 10.5 "Cheetah" is scheduled to be released in 6-months, and maybe we should wait for that...
...and so continues the infamous Apple infinite loop of operating system introductions.
So just buy it now.