I think lionfish are at least one of the top three threats to Caribbean reefs. And I have written several papers arguing that the threat from exotic species is often greatly exaggerated. (So I am generally skeptical about claims of pending doom from exotic invasions) In most cases, the addition of non-native species simply increases local species richness, with minimal negative affects on native communities.
There are certainly exceptions – exotics that become very abundant and competitively exclude native species or exotic predators that exert strong top down control on native populations, e.g., the brown tree snake on Guam, the cane toad invasion in Australia and rats on oceanic islands.
There is a lot of talk, particularly on the coral list about how the lionfish invasion can be halted and how they can be locally exterminated. Experience from other vertebrate invasions strongly suggests such efforts will be futile at best. But one glimmer of hope is that lionfish are edible, really easy to catch and they taste really good! Like chicken! No, seriously like grouper. One thing people do really well is drive vertebrate predators to near extinction by over-harvesting them! There are a variety of efforts underway to educate the public about how to safely catch, clean and cook them and to encourage the development of a commercial fishery. Some of this information is being organized and shard by “The Lionfish Hunter” at www.lionfishhunter.com (hat tip to Steve LeGore). The site includes recipes and a “lionfish hunter certification test“.