I finally got some time to try out my new Alvin mat and I LOVE it. It's heavy and THICK and so sturdy feeling, yet still flexible. I spent hours this weekend cutting on it, and not just with my rotary blade, I used a utility knife to cut out letters and you can't tell I even used the mat yet. The grid markings are a little tricky(they go edge to edge but also from the middle to each side, hard to explain), but I think once I get used to them they'll be very handy. It's black on once side and green on the other, so no matter what color fabric you're working with there's a contrasting background to cut on.
Overall, I'm thrilled with the mat so far, obviously I can't say anything about it's longevity yet, but I hope I can in the future.