Nate Silver at fivethirtyeight.com is a good liberal, but also a great numbers cruncher and he lets the numbers do most of his talking. In that vein, he doesn't see much upside for either Sen. Blanche Lincoln's candidacy or Bill Halter's challenge.
In summary, this is not a terribly good place for an ideological primary challenge. There's not much room to Lincoln's left in Arkansas period, especially not in a cycle such as this one. She has voted with her caucus reasonably often -- more so than someone like Ben Nelson or Evan Bayh. And the challenger, Bill Halter, is quite unlikely to win the general election.
The thing about this particular primary challenge, however, is that while the upside might be limited, the same is true of the downside because Lincoln is so unlikely to retain her seat anyway. Halter is clearly a smart (he's a Rhodes Scholar) and likable candidate and I can see why people would want to take a chance on him. But at best, this is perhaps the right challenge for the wrong reasons -- and at worst, it's a misdirection of resources that could be better spent elsewhere.