My take, for what it's worth:

I think prospects in this district are better for Democrats if Delay wins his primary but with a weak showing.

I think prospects for Democrats nationwide are better if DeLay loses it. That would show that the voters are taking the "corruption" issue very seriously. If an extremely powerful incumbent Congressman is abandoned by his own constituents, then the effects ripple out to a much larger group of races. Rank and file voters nationwide (in both parties) agree that corruption is terrible, but many privately reserve the opinion that nothing can be done about it. If GOP voters do something about it in TX-22, it encourages voters in many other districts to feel that it's an issue that can and should be a make-it-or-break-it factor in how they vote, because getting rid of corrupt incumbents can in fact be done.

So, either way it can be viewed as a gain, as long as DeLay doesn't win by a landslide.