Just spoken my mate via msn, he said he was also very sceptical but went ahead with the first session to see how it went. For him an improvement was made and visable within a week without too much pain, likened it to a strong sunburn like sensation.

They tell you the good and the bad in the first consulation and warn of problems blisters etc in the worst cases, this being dependant on skin types and locations of tattoo's.
I.e If your fair haired and very sensitive its more than likely going to hurt!.