Pros
I've really appreciated the scheduling flexibility, the supportive management, and the development I've undergone as a teacher. I've also greatly benefited from the opportunities I've had to teach and tutor on-site and online, learn different tests, and participate in content development and project management, among other roles. You are by and large working with very smart people, and that's also fun. Pay raises have been frequent and generous. To my surprise, Kaplan really cares about learning and the success of its students -- I wasn't expecting that from a test prep company.
Cons
As a new employee, your training and prep is paid at your area's minimum wage, and you may not get many classes to teach your first year, especially if you're in a small market. I don't think teaching for Kaplan is something to do if you need a job to pay the bills. As a long-term investment, however, it's been amazing. Teachers are rated based on student ratings, and once in a while you'll get a rating that feels (or is) very unfair.