I try not to go to far in customising my environment, simply because it leaves 
me with a more portable set of skills.  If I have everything I usually do 
aliased, and I switch to a different machine, how do I remember which 
commands are aliases and which are native?

This applies to Zscoundrel's comments too, if I tell someone the steps to do 
something, I can be reasonably sure they'll work as specified on a Mandrake 
or RedHat installation (which I will usually specify).

The friend who owns the OS-X box mentioned is of this mindset too - there was 
a fantastic environment file going around the UMKC VMS a few years ago, but 
when one of the less user-friendly admins started making changes to 
everybody's account it went away, and we had to learn how to do things in 
"real VMS" all over again.