I, like Cmo own the Achromat(3mm), Boston mat, and Herbies, however I also own the Harmonix TU-800EXi mat. The Harmonix mat is by far the best of the 4 but it will cost you $300. It seems insane to pay that much for a record mat but given they offer a 30 day money back guarantee, you have the choice to keep it or return it. I couldn't let mine go so now I have it at the top of the heap. It offers an element of presence that the others can't match....in my system of course but if you search the archives you'll find more info from fellow users.
http://www.combak.net/otherproducts/OTHERPRODUCTS.htm