So many of the bike that I ride come originally with the at81. This tire seems to break down very fast.. If feels good for the first couple rides and then feels like all the knobs are breaking off on a corner.
The m59 has much more longevity in the consistent response and solid feel after a season of abuse. This confidence from the tires response help make you a more confident rider. Again i ride mostly very inconsistent terrain of new england with rocks roots sane and very hard pack surface. The m59 is highly recommended by New england riders because of its consistent response. I basically ride this tire until the knobs start coming off and then replace it.
Here is a pretty good review from Kyle.. I disagree with his eval of the at81 for the reason stated above.. I think it feels washy after a couple rides..