The degree is popular because it enables people to study a variety of fields. Instead of there being numerous degrees for each specialisation (e.g. accounting, marketing etc) they all come together under one banner.
I've also met a few people doing commerce because they wanted to go to uni but had no idea what to do so picked commerce because it was a fairly broad degree.
Job opportunities will vary with what major/s you complete and other factors like marks, life experience, state of the economy, demand/supply in the industry. If you are interested in the types and availability of jobs that a commerce grad can do then you should search through the graduate opportunities section of a couple of university's career services websites or seek.com.au etc.
There will be maths involved in a commerce degree. Most uni programs have some form of stats involved. However the maths in a commerce degree won't be as taxing as the maths etc in a science/engineering/maths degree.
Overall difficulty of the degree will come down to you and what you are competent at. E.g. someone may find a maths degree easy and an arts degree hard and the opposite might be true for another person. It's really up to the individual.
