It's the best professional organization I've ever been a part of, by far. I urge you to join.
Reasons:
1. Professional development — at the summer conference, winter judging and regional meetings that take place around the country, APSE puts on workshops where sports editors and sports writers from shops large and small around the country share their experience, wisdom and best practices on topics both timely and timeless. I've never walked out of one of these without some new ideas that I could adapt in our department.
2. Networking — You will meet other sports editors and get to know them, bounce ideas off of them and pick their brains. I've never had an APSE member turn me down if I had questions or wanted samples of how they do things. And probably more importantly, I've seen numerous sports writers and sports editors get hired through relationships that began in APSE: most people are more comfortable hiring someone they have gotten to know better in non-interview settings. I met my current prep writer a few years before I had an opening at an APSE function, we sat down for 30 minutes or an hour. When we had an opening and she applied, I remembered her. I've seen people (writers and editors) rise from small shops to major metros through contacts made in the organization. There's a lot to be gained from actually knowing a hiring manager that you want to get in front of when your dream job opens.
3. Access — If you can make it to NYC in April, every year APSE has a two-day event where editors meet with commissioners of the NFL, NBA, NHL, MLS, NASCAR and others, lately also including the NCAA head. Depending on your situation, this can be extremely valuable.
4. Crisis help — You usually don't hear about it, but APSE intervenes several times a year when member organizations run into issues with the institutions they cover (colleges, pro franchises, etc.) to help settle credentialing or access issues. APSE can't make Nick Saban let you talk to freshmen but there have been cases where a particular writer or organization has credeials pulled over some spat or situation and APSE has helped get it resolved. Even if you don't want APSE to step in, there are people in the organization who can offer advice and even give you an idea of where you stand legally.
5. Olympic Access — If you want to send a writer (or writers) to the Summer or Winter Games any given year, APSE works with the U.S. Olympic Committee year-round to aid its members in getting credentialed, as well as to line up housing and such, wherever the Olympics are taking place. You stand a much better chance of getting credentialed working through APSE than if you go it on your own. (APSE also works with the NCAA on credentialing for the men's basketball tournament, and has an on-site person at each regional to help trouble-shoot any issues.)
6. Contest — This is the best and most important contest in sports journalism. Your work is judged against that produced by others in shops of similar size and resources. Those who do well in this get noticed. And if you participate in judging (which can also be done remotely if you aren't able to be there in person), you'll see great work and get a lot of ideas of kinds of stories that may be waiting to be told in your own backyard that you hadn't considered.
7. Diversity Fellowship — APSE has a program that allows a group (generally four to six per year) to get training and development over several months to prepare participants to become sports editors. The program has a great track record of fellows being promoted or hired by bigger shops, and all costs are absorbed by APSE.
I'm a past president, so I'm biased, but without question joining APSE was the single best decision I've ever made professionally.