Japanese business culture will undoubtedly shock the typical westerner. Aeon is not a so called "black company," but they do have high expectations and generally high-pressure, high-speed work environments. At times, especially as a new teacher, it can feel impossibly-paced. Be ready to quickly memorize a great deal of information throughout your first few months.
Management and teachers are hit and miss from school to school. I was quite lucky to have a good, experienced team. Unfortunately, some find themselves stuck with poor management or disagreeable head teachers. As a foreign teacher (and especially for Japanese teachers) it's easy to become unhappy in that situation. Staying focused on the students will provide a source of positivity and motivation. And keep in mind that managers are under tremendous pressure to meet sales and renewal goals.
You must sell educational materials. This is difficult for some teachers to swallow, but it is essential at most eikaiwas.
Overall, if you can take the heat, it's well worth it. The students are generally fantastic; you will build skills relevant to any career; you will get to explore Japan and meet new people. I have no regrets.
I would however, suggest looking into ALT and JET programs before settling on an eikaiwa. If you do go the eikaiwa route, Aeon is certainly among the best of the large language schools. Smaller local and neighborhood scale schools are also worth looking into.