At least McCain finally got an F-. It took them long enough to give him that. Bill Nelson and Jon Corzine deserves an F too, not NR. Snowe and Collins need F's. Baucus has been decent since 94 and I'd grade him higher than an F.
I don't pay much attention to ratings from NRA or GOA. C's, D's, and F's make me look harder at a candidate, but isn't the begin or end all. I'm on a state level PAC in Michigan. We had our first endorsements and didn't use grades. We did not endorse anyone(outside of Bush or judges) who did not turn in a survey. We also rated simply 'acceptable', unacceptable, or in rare cases, mixed. You can see how we did it at the link.