A Chiropractor has had the K-Laser for a month and I’m already seeing results far above and beyond what either a class IIIb or any other modality could provide.  I have used a class IIIb, I have ultrasound, and e-stim in my practice.  There is really no comparison.  There is some research out there that shows laser to be ineffective.  In these cases the reason was an underpowered class IIIb laser, or simply delivering too low a dose.
I agree with what Ed said about power and dosage.  And I’ll add that if you don’t have enough power, you won’t be able to treat deeper structures.  If you have an underpowered laser and you are able to get it to the injured tissue, your treatment times (to deliver an effective dose) could be ridiculous (30 or more minutes).  On the flip side, there is such a thing as photo-inhibition.  Deliver to high a dose and you will actually slow the healing process.  Maybe this is what happened with the Apollo laser and the TMJ that got worse.  The apollo is low powered for class IV, but that region of the body doesn’t take much power do reach an appropriate dose.  I would think that around 4J per square centimeter would be enough for a chiropractor.