I usually don't stay up to date w/NT, so the only thing I know is that sp4 is pretty stable. As for not upgrading, it's my supervisor's choice, not mine. I can present a case, but I haven't really studied sp5 yet (as in, what the differences are and what it will do to our systems). Lastly, There's about 60 computers and it'll take a while to get them all done.
In my opinion, yeah, I'd like to upgrade and I'll come in on the weekend (w/overtime pay, of course!) to put all the machines on sp5.