I've been the Integrated Library System (ILS) administrator at my library district since 2005. I started about 6 months after the district had migrated from Dynix to Horizon. Almost a decade ago I installed Koha on my home computer. However, other than that brief episode, I only have experience with Horizon.
There are many things I dislike about Horizon but there are also a number of features that I think are great. For instance, I love that all my data is in a standard database. It sounds like such a simple thing but in the library world it's best not to take the simple things in life for granted. While an end of life date hasn't been set for Horizon, it's no longer in active development*. However, at some point, our district is going to have to migrate to another ILS.
A couple of days ago I was thinking about the prospect of moving to a new system and all the new ways of doing things I'll have to learn. I decided that I needed practice with a wider range of library systems than I currently have. As a result, I've decided to install Evergreen on a home computer and play with it a bit. I hope that learning a new ILS is like learning a new language. The first couple are hard but the more practice you have the easier it gets.
Why Evergreen? It's extremely unlikely that our district would migrate to Evergreen. However, it's open source so it's relatively easy for me to get my hands on it. Also, within the last couple of years, I've seen more large systems migrate to Evergreen than Koha, the other large open source ILS. If my Evergreen experience goes well, I might decide to dabble with Koha also.
* That's the official statement but recently we've been getting bug fixes released more than once a year as well as getting international customizations integrated into the main code base.
