I'm in UXPA because a lot of my colleagues and professional contacts are too. Other things that I find both interesting and useful are the annual survey results and the excellent UX Magazine. Depending where you live, their conferences and various events are great too. Local chapters are one of the most popular aspects, but many areas don't have one so your mileage may vary. The UXPA is not very active here in Melbourne.
The UXPA renamed itself (from UPA - Usability Professionals Association) and relaunched their website in 2012. It has a broad view of design that also incorporates IA, UX, Usability, SEO, etc, etc, and seeks to be an umbrella organisation for a range of UX-related specialities.
It's often compared with the IxDA (Interaction Design Association), a similar professional group with an intentionally specific focus on Interaction Design. Sometimes UXPA and IxDA co-exist in a city, sometimes one of them is the only presence. It's not uncommon for people to be supportive of both. There's a great articulation about the similarities and differences on Josh Seiden's blog.
Personally I get more practical value from IxDA and it's community, but I like the professional recognition from being a UXPA member.
If neither UXPA or IxDA are present in your town/city, some alternative options are:
IAI - Information Architecture Institute, if your interest is strongly in the structural design of shared information environments (especially websites, software and intranets).
HFES - Human Factors and Ergonomics Society. Mainly in USA, Canada and Europe.
AIGA - (previously called 'American Institute of Graphic Arts' but now just AIGA). If you have a formal design background and live in the USA, you'll fit in here.
SIGCHI - One of the Association for Computing Machinery's special interest groups, and a leading organisation in Human Computer Interaction (HCI). Very ergonomics focussed.
IEA - International Ergonomics Association. A federation of more than 40 individual ergonomics organisations around the world. It supports ergonomists and ergonomics in developing countries.
- ]If you're in Australia, also have a look at CHISIG - a special interest group under HFESA, although it's really not very active any more.