Our library switched to the Apollo ILS over four years ago from a legacy system. Migration was about as painless as any migration could be. Having gone through two separate migrations with other libraries I know first hand how difficult they can be.
Customer service is always very helpful and friendly and if they cannot address a problem on the first contact (rarely) follow up has usually been within 24-48 hours. They are continually adding new features which always help staff do their job better.
Regarding costs versus features; Apollo does not have some of the sophisticated features of the enterprise class ILS systems used in large public libraries. That said, Apollo is significantly less expensive and for small to mid sized public libraries those high end features would most likely not be utilized.
I have used six other ILS programs in my career, all of which did an acceptable job. For our application Apollo is by far and away the best choice for both cost and features. Other ILS systems for our sized library are good (and I have used the main competitor), Apollo is better.
One of the biggest issues we have is getting our community to use the library. The online catalog, database integration, and most particularly a smooth transition to our downloadable E-book and audio book platform has attracted new users. Prior to the switch to Apollo the library's catalog records were variable from barely adequate to downright bad. Access to free, high quality copy cataloging records as part of the package has helped us improve the library's catalog and given more access points for end users utilize.