Jason,
There will always be cases where certain individuals either have higher limits or no limits, that's to be expected.  For those individuals you will find that they most likely have a higher I/O demand than their typical end user since they store more data and access more often.  The key thing to remember when dealing with large limits is to distribute the data across folders rather than storing it in a single folder; the more data within a folder the more processing the server (online mode, OWA, etc) or the client (cached mode) has to perform in order to display the data (e.g. user sorts 5000 item folder by subject line).  The time it takes to complete could be perceived by the end user as troublesome.
For those instances when users believe their performance is lacking, I would recommending using ExMON and ExPTA to help diagnose and uncover the potential problems.
Hope this helps.
Ross