Bad clients are bad clients, but speaking from experience, it can be taken to an entirely new level when the bad client in question happens to be a law firm. Some of the less savory high volume practices can be especially bad, and they all go out of their way to project an air of competence and professionalism that disappears pretty damned quickly. Personally, I think part of the problem with those guys is a serious sense of arrogance and entitlement, as if their being a law firm will force you to go away despite not being paid. And if you don't, well, they can always play games to delay and string you along. After all, you're paying for representation directly. They can at least swallow the opportunity cost of dealing with you somewhat easier.

Nowadays, if I'm contacted by a law firm for work, I look into their reputation very carefully. In my experience, corporate law firms--regardless of size--aren't a big deal compared to some of the high volume personal injury/immigration/divorce/etc. firms. And yes, I realize I'm doing a lot of generalizing here.