As 'Meldrew' mentioned, the Horizon is indeed set to join Island cruises next year after re-fitting.
I am a travel manager in UK and have watched with great interest the joint RCI/First Choice farce they call Island cruises. We even made a documentary over here about the first few months of the ship!
Island cruises have redeemed themselves somewhat from the disasters surrounding their first season (or so im led to believe!) but they are very much considered the 'poor-mans' cruise, with simple, small accommodation and school canteen style buffets.
There are similar cruise itineraries on the likes of 'Thomson' and 'Ocean Village' but with far, far superior ships.
The Island Escape their first and currently only ship is ex-RCI, but it was old-school RCI and was originally converted from a cruise-ferry, so it has always lacked style and character and is just there to serve a purpose and the 'pile-it-high, sell-it-cheap' masses who want cruise but not at cruise prices. However the moral of me adding this comment is that i have always been a fan of Celebrity and cant help but think that whatever your opinion of the Horizon, that it is a waste sending it to Island and that they will inevitably ruin a perfectly good cruise ship in doing so.
It may have had its day but with the right company to take it on would have many, many years of profitable sailing ahead.
We'll have to see what happens... watch this space!