There is a whole thread about 10.5.8 already. Take a look :

http://www.infinitemac.com/f36/10-5-8-is-out-t3554/

Generally it is safe to update. It works fine with 9.5/9.6 backed up kernel, or vanilla on intel hardware.
Still...some people reported problems with sleep, sound and time machine after the update - so be careful.

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