I'd definitely give it a good thought before sending money. After I wrote back explaining my problem,
Sintech claimed that the membranes for Spectrums work just fine and that I should re-check if I installed them correctly. I've spent a few hrs playing around with it and made a conclusion that everything was in fact installed fine and that some keys simply wouldn't work smoothly or at all. I think that the production of those new membranes was a rush job, and Sintech doesn't want to admit it. I've also seen similar reports from other people claiming the same thing as I, in regards to the membranes.
So my experience was kind of negative, however I'm not sure if the same applies to all of their products. It definitely gives you a heads up that you are dealing with incompetence, in my opinion.