I gave up on C4L a long time ago, when they started soliciting donations to fight the healthcare bill. At the time, I did think that there was little reason to defend a multibillion dollar insurance industry with my own limited money. I probably should have anticipated that they would just turn the bill into a handout to themselves as usual, but still, there were so many voices on this issue that I didn't think it was necessary. Once the insurance companies turned it into a handout, it was a done deal anyway. When that much money is involved, politicians care less about principle and more about what they can personally get out of it.
In any case, it doesn't really matter. The people who were involved with C4L before and have lost faith in the organization aren't just going to disappear, they will just direct their efforts in ways they think will be more effective. The only problem would be if people quit C4L and just stopped paying attention entirely.
This is always going to be a problem with an organization like C4L. You can't be all things to all people. If you think C4L is doing what you would like them to do at least most of the time and they will be effective, then there's no reason to quit. Any organization with such broad goals and activities is always going to do something you don't agree with.
The effectiveness issue is key. There is an economy of scale involved in politics, and having a large, central organization has many advantages over small, single-issue groups. If you broke down C4L's issues and formed a single-issue group over each one, none of those groups would likely be able to gain as much ground.
Also, you don't have to "quit forever." Everyone of course knows about the idea of "voting with your wallet." If the organization as it is operating today is doing a good job, then donate. There's no reason to hold a grudge about something they did in the past if they have changed it. Likewise, if they're doing a terrible job now, then it doesn't matter how much good they have done in the past.
I'm not "done" with C4L but I'm not putting my money and effort into it at the moment. I did in the past, and may do so in the future. Especially at the moment, I personally believe it's more important to help out with the campaigns.