My home computers currently run Red Hat Linux 7.3 “Valhalla”, except for my router in Orsay which is still on Red Hat Linux 7.1 “Seawolf” (see here for explanations on Red Hat release names if it sounds very mysterious) and a lone FreeBSD box somewhere that doesn't do much but sit idle (an archaic computer with an archaic version of FreeBSD, anyway). This can't go on forever, though, because Red Hat 7.3 is now getting quite old: lots of software pieces presently require newer versions of this-or-that package or library; but, more importantly, Red Hat is only maintaining these versions until the end of the year (as far as security vulnerabilities are concerned, I mean), so I'll have to consent to change before the end of December.
What I'm worried about is that this upgrade will not be as seamless as I wish it to be. Every system upgrade entails changing the version of hundreds of programs and libraries, sometimes in a not entirely forward-compatible way: configuration file formats change, programs behaviors vary. As usual with these things:
The first 75% of the work take 75% of the time.
The last 25% of the work also take 75% of the time.
(And so on recursively. 
) So, typically, installation
proceeds without a glitch, and almost everything works fine.
But there's always a dozen irritating quirks: maybe the Mozilla fonts
are suddenly ever-so-slightly wrong (or aren't anti-aliased); maybe my
ICQ client randomly segfaults at start; maybe Emacs
suddenly decides to handle accented characters in a strange way; maybe
Perl prints a warning message that I can't understand upon launching
certain scripts; maybe my Web server refuses to run
cgi-bin scripts; maybe my NTP timekeeper
refuses to synchronize on the prescribed servers; maybe control-C
mysteriously ceases to function in console (text) mode; maybe
Xvideo won't function; maybe ssh keys just
won't function; maybe my PostgreSQL database doesn't
handle permissions properly; maybe Gnome refuses to take into account
this-or-that keystroke shortcut; maybe my printer daemon refuses to
let distant computers connect. Or something. There are millions of
things which can go wrong, and almost none of them do. But every time
I've upgraded my Linux installation, I've encountered a few of these
microscopic problems which rarely cause any serious damage but just
irritate you endlessly while they aren't solved—and take forever
to solve if you decide to discover the deep reason for this-or-that
misbehavior. And since I haven't upgraded in a long time, I fear I'll
encounter a good number of such glitches next time I do.
Hence the reticence. But there is also the following dilemma: Red Hat or Debian? So far I've always been faithful to Red Hat, but their increasing commercialism, and a couple of not-too-easily forgivable problems (and acts of pure stupidity), have tempered my good opinion of them. So, should I upgrade to Red Hat 10 when it comes out, or, uh, sidegrade to Debian something-or-other? But there are things that worry me about Debian, also: their “stable” release is unreasonably archaic, for one, and I'm not sure how security-aware their “testing” release is—but I want both the latest bells and whistles and a reasonable tracking of security and stability issues. I also consider the Gentoo Linux distribution, but I've heard mixed reports about it.
Without doubt, computers can consume an unbelievable amount of time needlessly.