That's the rub, isn't it? I think it will be different for everyone, depending on how demanding their site is, how critical it is, and their personality profile.
It's worth it to me because my sites aren't demanding and are somewhat critical - that is, being down has some monetary impact to me.
The sites I care about are fully cached WordPress sites and neither NFS nor MySQL are bottlenecks for these sites. I can't speak for any other web applications.
I'm still evaluating PS. So far it's been an improvement, but things were pretty good for me before so it's not like the difference is like night and day.
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