They are not illegal, you are not changing emmissions hardware, only tuning your engine, and in fact generaly improving emmissions. Legal in all 50 states.
Especialy usueful if you change tire size (which I want to do), as well as helps to program the best fuel/spark settings all along the RPM band, as well as fine tuning your shift points.
I have seem them work and add value, just can't find one for this TB.