I changed the transmission fluid today. I've never done it myself and per all the instructions and forum threads I've read online I made the requisite mess.
The tube on the pump I used popped off as I was pumping the new oil in, and it sprayed everywhere; all over me and all over the underside of the car. Next time...
The last time the fliuid was changed was about 5000 miles ago but it was in 2009! Definitely due as I felt shifting was getting rougher and rougher. I was worried that I had perhaps waited too long, but the magnet on the plug has just a little bit of material on it.
Now that I've done it myself, I will do it at least every 24 months as recommended.