I find Senator Obama's decision completely defensible and unsurprising. The system is broken. We cannot expect opt ins by successful candidates, especially in the internet age which has greatly decreased the cost of fundraising from micro-donors.
Common Cause says it won't criticize Sen. Obama but nonetheless gives him a "demerit." (What, is he back in second grade?)
The Public Campaign Action Fund is supportive of the decision.
The Brennan Center (in an emailed statement not yet online) is noncommittal.
And the Campaign Legal Center, in the past closely allied with Sen. McCain and headed by Sen. McCain's counsel, Trevor Potter, is silent.
UPDATE: I've been updating this post, along with an offer to moderate a discussion between Trevor and Bob, here at Rick Hasen's Election Law Blog.