I wouldn't do it for a few reasons...
Firstly I'm not an angry person, I'd probably have a cry and a whinge before I actually dobbed someone in.
Second, they probably don't always get shit gear, so they may be useful to you in the future. Example, If they sold you White Doves (piperazine) you can ask for your money back, and even if you can't get your money back, you'll remember not to buy the same pills again.
Or you can just not buy from the same guy on principle. Easy.
Third, you dob them in, they may dob their supplier in and the cops might work their way up the chain to the point where you could be getting some really decent pills out of our mouths and off the the streets.
If you were sold something adulterated which could be fatal (PMA). Post a warning on Bluelight and Pill Reports. Tell you friends. Tell the dealer. Tell some randoms in the smokers area.
IMO, not worth your time and effort to dob them in.