my favorite thing in the world is a stormscope... but the problem with the radar function is that it is prone to black-out more significant stormy weather behind initial echoes. If you see a light return directly ahead, you do not know if behind that "cell" there is another, possibly stronger area of precip. It requires some inference, but the best recommendation is to just stay away from the precip, and not fly around it: you could be " coming around the rain bend" and find yourself flying directly towards a new echo, and thus another area of possibly stronger precip.
The ones that determine lightning strikes don't have this weakness, but then again they don't show the precip either. They do tell you where the storms are though...
I think some of the new GARMINS have a combination of the two... I'll have to do some research and get back to you.