Black locust is a hard plant to call either good or bad, it really depends on where it's at and your intentions with it. They are great companion plants for walnut trees as they provide nitrogen to the walnut trees and are not affected by juglone. Juglone is a chemical which occurs naturally in all parts of the walnut tree especially in the buds, nut hulls, and roots of black walnut trees. This chemical kills many other plants that are sensitive to it and unfortunately there is no cure once plants are affected. When planting black locust as an under story tree many walnut growers have found they get higher yields of nuts without any supplemental nitrogen needed.
As stone said black locust post last virtually forever and make great poles for grape trellises, fencing, and many other things but at the same time if you don't have a large field or area to grow them in beware the warnings Alyssa Blue said. They are hard to get rid of once established and do sucker if roots become exposed or are damaged. Also the roots will enter your septic lines and will damage your homes foundation if growing in close proximity. I wouldn't recommend growing it unless you have the room for it. if you live on just an acre or less forget about it. In the long run you'll regret keeping it no matter how pretty the flowers are or what benefits it gives you from producing nitrogen.
When you do remove it or transplant it be sure to wash your hands afterwords. the leaves and bark are toxic to not only us but to animals as well. If you have horses or livestock make sure not to relocate it near them or they could die from cardiac arrhythmia .
