I agree. I had bought some butyl tape from an RV store (on Amazon, I think) and it was kind of OK. Then I bought from MaineSail and I much prefer that one. A roll lasts a long time but when I run out I will surely buy from him again.Yes, you should. Maine Sail who started this thread, sells it on his website. The link is in the OP. Not only are you assured of getting the proper quality, but I think he deserves the business for having provided the free value of the instruction in this thread.
I've purchased several rolls from him. I admit, I did purchase some hardware store variety to make a temporary sticky plug that I could remove on a project. It was intentional to have something cheap and disposable. It was clearly an inferior product.