I have found a lot of value in SAGE, USENIX, and LOPSA, primarily in the degree of professionalism, technical skill, and support within the community. In 1998 I moved from being a statistician and web developer for a large dot.com to being a sysadmin and had the great fortune to have an employer that would pay my way to LISA. In 2001 I presented a paper on reliability modeling; the next year I was on the LISA program committee. At present, I'm on LOPSA's Board of Directors.
Speaking only for myself, your membership in a professional organization is what you make of it. The national organizations may focus on issues that seem much higher level than the focus of the average sysadmin, but in some ways that's their purpose - to look at how other professions have developed, to find ways to raise the standard of performance of the profession, to focus on notions of ethics and standards of conduct, to show the state of the market (I'm thinking specifically of SAGE's annual salary survey), etc.
My feeling is that the bulk of sysadmins are in your position - it's unlikely they'll fund a trip to LISA even though the "hallway track" is well worth the price of admission. And again, not stumping for my organization specifically, there's a great value in organizing your local sysadmins. I suggest informal dinner, drinks, and "recovery" (aka cathartic ranting) to start with. I know from my experience with Austin's LOPSA chapter that just getting a few sysadmins around a table with burgers and beer does wonders for establishing a community without flying halfway across the country to the Big Conference.
Rarely are the sysadmins in competition with each other, even if their employers are. Usually people are very willing to help each other because there's a sense that we're all in the same boat with regards to management, budget, users, vendors, software, etc. And when push comes to shove in the market, it's better to have contacts who know you and your skill level and your personality before you need a job than after. IMO the national groups should foster the development of local groups, I have championed this within LOPSA though I have not been as successful as I would have liked and I blame nobody but myself in that regard. Regardless, you don't need a national organization to build a local community. Find four people, meet regularly, and build up as you can. Where a national organization can help is to contact members that live near you and help build your local. They can also provide speakers and suggest presentation topics and lessons learned from other locals. But ultimately what you get out of the organization is what you put into it, whether that's writing a paper, volunteering, forming a local organization, or sitting through interminable teleconferences trying to set national policy and administrative hoo-hah ;)
The Big Conference is but once a year but your local peers are near you all year round. While I may be an advocate for the national organizations in general and one in specific, I strongly encourage you to work locally to develop a community that supports each other and raises the standard of excellence of the sysadmin profession. If you have to choose between action and membership, choose action.
Again, this is my personal opinion, not that of LOPSA nor the LOPSA Board. Apologies for the disclaimer but you know those lawyers and their picky "whereby"s and "heretofore"s. :)