I have nothing but good things to say about the Masterpieces I have (or have experienced).
Optimus is fabulous. Grimlock is fun.
My 8 YO son got Ultra Magnus for Christmas and LOVES him. He now wants a MP car (or 4!) for his birthday to fill UM's trailer with (at the moment he is using CW cars). UM is really nicely detailed and surprisingly my son had no problems transforming him whatsoever.
I think Gok's idea of meeting with someone who has some of the MP's you are interested in so you can check the real thing out is a really good idea. Not all MP's are created equally and at the end of the day I think rather than focussing on bad reviews, unless the vehicle you want has a critical flaw, you shouldn't write it off without seeing the real thing first.