I think it depends on the reason that a department doesn't have a chair.
If they don't have a chair because they are in the middle of an active search process, than that is not neccessarily a bad thing, you just need to know the parameters of the search.
For example, as I understand it, both University of Utah and Spaulding/Harvard are in the middle of current chair searches. These are both good, albeit young, departments, and while you can never be certain how things are going to work out, all indicators is that these are both programs on the upswing, and as such should be in ok shape.
As another example, last year the University of Miami was in the middle of a chair search. They brought in Diana Cardenas, and are now a better program for it.
As another example, the University of Washington has an interim chair at the moment. This is a department with an outstanding long term tradition, so the assumption on most everyone's part is that it will continue to be a strong department throughout the transition.