I am a CMA, now working as a unit secretary while I'm in school for BSN. I got hired by the clinic where I did my externship and LOVED it. If I hadn't had to leave in order to go back to school, I definitely would still be there.
A few things:
Pay is not great, but not terrible. I worked in an ortho clinic in a hospital (one of the lowest paying places) and started at 11.50/hr.
Hours are great if you are looking for a 9-5 type job. I worked 8-430, M-F, all holidays off. But it can be difficult to find work as a CMA if you need an evening or night shift job. They are out there, but they are few and far between.
The job is actually really interesting, especially if you work in a surgical or specialty clinic. You learn a lot if you work for a good group of practitioners, and you actually have quite a bit of responsibility. Because of the way our units were structured, I worked 3 days a week in ortho and 2 days in burns. I learned all about different treatments and surgeries, wound packing and dressing changes, etc. We would occasionally float to the surgical unit where we would assist in the setup of PICC and central lines. I won't ramble anymore, but it was really, really interesting.
However, if you are planning to continue on to nursing, I would get a CNA over a CMA. There are just way more positions you qualify for as a CNA, and it's usually a shorter and less expensive program. In ambulatory, you can do a lot as a CMA, but in acute care, we don't qualify to do jack when it comes to patient care.
If you can get in to a community college CMA program over a private school, I would do that, as it is much less expensive.
Don't put off taking your phlebotomy certification test, because this can create a lot of opportunities for you.
If you are looking to advance your career, or go back to school, or in any other way progress past being a CMA, I would try to get in at a hospital for your extern and do your best to get hired there right after you graduate. They usually offer tuition assistance and/or support and other really good benefits, and it is so, so much easier to get a job in the hospital once you already work for them.