My experience with Shinkos is that they grip really well, but they wear down really fast. I only got about 4000 out of my last rear, and 6k out of my last Shinko front.
I've heard people say they've gotten as much as 7000-8000 out of a rear, but never any more than that. Considering it ends up costing me about $160 for a mounted rear Shinko, and $200 for a mounted rear Michelin Commander II that yields 15,000 miles, I'll never buy Shinko again.
Even if you mount the Shinkos yourself, you'll get double the mileage and comparable grip for $40 more with an alternative. Some people swear by them, but with tires being so critical to the safety of me and my wife, I'll pay a little more for the added peace of mind.
Ben C - Cary, NC
2001 XV1600A Road Star
2003 XVS650A V Star Classic