I think this is one of the many cases in this part of the hobby where the quality of the kit really depends on what the builder wants from it.
It is indeed a modified version of the Kearsarge kit. The latter does a pretty good job of representing the real Kearsarge - as she appeared in the 1880s. (She'd been modified fairly significantly since the Civil War. The article on the Steel Navy site does a terrific job of explaining the changes - and the various detail problems with the kit.) The Alabama kit has a different hull - but not much different. It uses the same deck components, and many of the other parts are common to both kits.
In Revell's defense, at the time the kit originally appeared (in the early sixties) not much was known about the real Alabama. (Experts on the subject at the time could take issue with some of the kit's features, but it looked pretty much like the drawings and paintings of the ship that appeared in most readily-available sources.) But during the past forty years a surprising amount of information about that ship has come to light. We now have access to some pretty good plans, a builder's model, quite a bit of good documentary information, and even a couple of photographs that show the exterior of the ship. (Even in the sixties two on-deck shots had been published frequently.)
In 2002 the Naval Institute Press published a book called C.S.S. Alabama: Anatomy of a Confederate Raider, by Andrew Bowcock. It's a remarkable publication, containing all the known photos and artwork and a set of detailed plans. Here's a link to it: http://www.usni.org/webstore/shopexd.asp?id=18770 .
The plans, photos, and other material in that book make it pretty clear that the ship differed pretty significantly from the Revell kit. A Civil War buff wanting a really accurate model of the Alabama probably won't find the kit satisfactory. The fact remains, though, that it's a big, impressive, fun model. Such things as the gun mounts, the gear for hoisting the propeller (working!) and the delicate "bridge" spanning the bulwarks amidships are just about unique in the plastic ship kit business. And the crew figures (recycled from the Cutty Sark, but who cares) are wonderful. It may not qualify as a genuine scale model, but for Olde Phogies like me it's a terrific exercise in nostalgia.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.