Bridgestone makes great tires! The BT015 is a great street performance tire. They warm up quickly and on a warm summer day, you can lean your busa over until the hard parts get scuffed from asphalt. Even the older version (BT014) was a great tire. I went through a number of sets and was quite pleased.
The BT021 is also a very good tire but is meant more for touring. Although it has plenty of grip for the everyday rider, it's a bit slippery if you are a knee dragger. The 21 will give you much better mileage than any performance tire.
I am a bit partial to the Dunlop Qualifiers and the Pirelli lll's myself. Both give outstanding performance at the expense of high mileage.