Siping is cutting across the tire most of the way down and about 1/4" apart. Tire dealers are familiar with this process. It allows the tire to get better traction by opening up as it meets the pavement. Siping does lower the amount of miles you get out of the tire but the benefits outweigh that especially if you live in the north where winters are long, icy and snowy.
I run aggressive tread year round no matter what, and I keep a set of tire chains for when it snows - not those cheap ones either - I like the real chains!
I looked at my tires this weekend, realizing that I put them on at about 65k, and I now have 135k (70k on tires for the math impaired). They look like they still have about half the tread on them. I was a little sceptical about the Toyos because the initial tread depth wasn't very deep, but they are the best tires I've ever had. Maybe the siping extends their life, maybe not. The sales pitch was that not only do they grip icy and wet roads better, but that the sipes keep the tire cooler in hot months.
I don't often buy the same thing twice because I KNOW the grass is greener on the other side of the fence, but I'll get another set of these, siped, when I need to replace them in a few years. Unless some new miracle tire material that doesn't wear comes out, these are the best tires I can think of.
Note for snow/ice dwellers: get a set of tires, all 4, that are studded. Pop them on when the roads turn bad, take them off at the end of winter. having a rotating lineup of tires will make your purchases farther apart. Studs all the way around help your handling as well as your safety. I lived in Breckenridge CO and Bend OR, so I know.
*yes, use real chains, wire chains are only good for packed snow and ice. the GoClaws are made like crap since the company was bought out.