Im currently doing my APEX in an office in Houston. Your experience is completely dependent upon the dentist you are working for. I was looking for somewhat of a break this summer, but it seems like my dentist will have me working pretty hard. Basically, my job is to set up the operatories, take radiographs, assist, breakdown the operatories, take impressions, probing, charting, etc... Basically, everything but the drilling and filling. The hygienist is going to train me to do some of her work. Its a good experience, however, I think 3 months may be too long. I see this being beneficial for about 4-6 weeks and then it will probably get repetitive...very much so. He will also send me to run some rotations at specialist's offices, like endo, ortho, omfs, and perio. That will be nice change of pace which will help keep things from getting stale.
In any case, Im more interested in the business side of how he runs his office. All the other stuff, I will learn in school. However, the business aspects are learned in the real world and I may as well get an early start while I have the opportunity. I think getting involved in this area will be the most beneficial part of APEX, at least for me.