Even if the NRA is just lobbying for the NRA, as opposed to lobbying for NRA members, the things they do in other areas still makes them worthy of support.
Whittington Center, National Firearms Museum, various educational and training programs, all worth the cost of membership, even if the lobbying aspect did not exist.
My political action money goes to SAF or GOA though, JFPO is another good Organization. they are younger, far more aggressive organizations that were founded far more strictly as "Political Action" groups. While the NRA, founded in a time when guns were more normal than shoes, was formed to fight the threat of loss of interest in shooting, rather than any threat to shooting Rights..