I'm a member, have been one for many years. I spent 24 out of the last 25 years on active duty (retired last July) and being a member has helped me join in with the local modelers whenever I get restationed.
One of the benefits to being a member is that you get a pretty nice bi-monthly magazine called the Journal. If you consider your membership dues to equate to the price of a subscription to the Journal, it works out to about $4 an issue (6 issues for $25/yr). The IPMS has its own website and forum much like this one but not nearly as active.
http://forum.ipmsusa3.org/
A good number of members of local IPMS clubs I've been a part of are also national IPMS members, but it isn't required. You can be a member of a local club without being a national member and vice versa.