No prep courses, I only went through the ACER practice test once and one of the other practice booklets maybe twice. My score was disappointing at 158, the 67th percentile (i.e 67% under me) [But I was pretty satisfied in that the amount of guessing, the amount I didn't finish - some people must have done *really* bad]. To put it in perspective I think the UWS/UNCLE cut offs are around 183-7 or something like that, and at UNSW to get an interview my UAI would have to probably be 99.5 or something.
Some people can get very high UMAT scores with no preparation, however as much as they lead you to believe it is a test of 'natural ability', the fact that there are stories on MSO of people scoring >200 after Prep courses compared to 150 earlier - shows that you can improve.
I'm still deciding whether to take a prep course and try again next year for UWS/UNCLE. I heard MEDENTRY was good - keep in mind as far as I understand it is a very expensive way to basically just get a bunch of practice questions and good answers. {See, the ACER practice books have answers "A, B, C or D" etc.. but no explanation so it can make it slightly pointless}