The idea that the NRA is a tool of the gun manufacturers is nonsense. NRA has frequently and repeatedly taken positions that were in conflict with the best interests of gun manufacturers. In 2004, they killed the Protection of Lawful Commerce in Firearms Act that would have benefitted manufacturers rather than see anti-gun legislation that didn't hurt manufacturers be attached. As recently as 2013, the NRA opposed legislation that would have provided a new revenue source for FFLs and ended most private sales.
As far as being a Democrat, the NRA welcomes all groups. Former Dem Senator David Boren is a current member of the NRA Board of Directors. Now retired Rep. John Dingell was a former NRA Board of Director member. Former Dem. Senator Zell Miller was a keynote speaker at the NRA convetion (as was Boren). Sen. Heidi Heitkampf is NRA endorsed, and the NRA has supported numerous Dem Senators in the past. In fact, one common complaint among Republicans is that past NRA endorsed Dem politicians like Joe Manchin, Kristen Gillibrand, Al Gore, Bill Clinton, etc. have a nasty habit of turning on the NRA when the chips are down or when they reach national office.
On the other hand, you have gentleman like former Dem Senator Max Baucus, who stood solid on the Second Amendment under immense pressure last year even though he was retiring and could have easily sold us out.
The NRA teaches gun safety, education, and provides the infrastructure so that people have places to shoot safely and learn the safe use of firearms. They don't actually lobby Congress with your membership dues (and their charter prohibits it). If you want to promote gun rights, the organization you want to support is NRA-ILA (lobbies Congress), NRA-PVF (elects pro-2A politicians), or NRA Civil Defense Fund (helps pro-2A litigation). However, without the NRA, we wouldn't have any of the other organizations.
So yes, I think it is a worthwhile investment. You should understand that Dems are underrepresented in the NRA; but considering the Dem national platform on the Second Amendment, they still remain well-represented in the NRA (as recently as 2010, the NRA ran a piece in its magazines highlighting Sen. Harry Reid's help in building a range).