I'm doing UMEP (University of Melbourne) this year and I really enjoy it and it also supplements your other maths subjects so I would definitely recommend doing it.
Some of the stuff you learn extends from topics covered in Specialist, but there are some brand new areas that are really interesting.
You have two exams, one 3 hour exam and one 2 hour exam with no notes or calculator for both, but the exams will be probably a week or more after your last VCE exam so you shouldn't have to stress too much, just keep up with your work and assignments during the year. Also, the workload isn't too much, you have assignments which must be done, but other than that it's just about consolidation of what you've learned, doing as many (or as few) questions as you feel you need to, and just generally thinking about the implications and uses for what you've learned.