If it doesn't do what is says on the tin then it deserves critical comment, the level of negativity can vary considerably though and will depend on the expectations beforehand. If you think it's shit as you walk away then it's shit and doesn't get better with time.
Services promised but not delivered is a common theme and probably the most common cause for complaint, choosing to overlook this and give a Pro$$ie positive feedback is total nonsense and will likely lead to disappointment for another guy. At least saying nout wont help a poor provider get more business.
A lot of girls over-inflate their skills and abilities promising the 'best BJ in the world' for example so use a bit of common sense, but if she turns out not to provide this or that, suddenly charges extras, is unclean, the place is a shit-hole, displays questionable behaviours or does not give the time agreed then others should be made aware.
JRC is spot on with first impressions and if in doubt walk whilst you have the chance. I had a bad one in April, the place was a crap hole and it just got worse from that point on. I should have done a 360 and gone home with cash still in my wallet.