I find the minigun to be very useful to finish players or even vecs with low health.
If someone starts trying to kill me with the mini, I raise my shield gun, I retreat, and I wait for other teammates to join the fight. Or maybe I charge him with the shield gun to seek for a gloriuos death. It depends on my mood xD
If two or three enemies do this to me, then they are doing some teamplay and so, kudos for them, I think

(even if it's annoying xD)
Also, sometimes I
need to use it to kill very agile players and/or players way more skilled than me *coff coff funk coff coff* (*)
However, in this case, every time that I have to choose to shoot with the minigun, I consider it a failure for me as a UT player. It's like saying "Ok, I recognize that you are better, in an ideal world I'd practice seppuku to myself, but I have to kill you for the sake of my team anyway).
(*) actually, I think that I didn't shot Funk with the mini in a long time, basically since we have our shield thing going on xD