Some of your clients scripts may not be compatible with php5 therefore breaking them. Personally I'd wait till php5 is more compatible with cPanel and before you perform the upgrade notify all your clients to ammend their scripts.
Cut short; I don't reccomend it
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If a script doesn't support php5 after a year of RCs and almost a year of "stable" releases, it's time to cut your losses on the script and just move on, because obviously they don't know what they're doing, or don't care about their customers.
As far as php5 issues:
I've used php5 since the day it was released as stable , and have had absolutely no issues whatsoever. I've used it both on DA and CPanel. of course, I don't use the standard CP compile script (much), I've got my own written, but it's not given me any problems at all.
php5 is very stable. Anyone saying it's not obviously hasn't had a bit of experience with it .
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