Lionfish are great to eat. I've had them on a few occasions and like most white meat fish, it doesn't have that strong "fishy" taste to it.
The lions are a huge problem in the Caribbean. I've only been to Grand Cayman and to Belize, where there were much larger numbers of lionfish present. Every dive in Belize we took spears with us. Most times we would get a couple, but they would be smaller. The larger ones would be deeper where I wasn't going. On many occasions nurse sharks would follow us and eat the lions right off the spears.
http://www.youtube.com/watch?v=yyktW-vO3kQ
This is a problem that will never be fixed. It is too widespread and it will be a matter of time to see what affects there are on the ecosystem.