I spent some time there this spring. I took my cyclocross bike and a set of durable road tires. Couldn't have been better. The 'mountain bike' trails are jeep roads. Decent road riding and a few short grunts for hills. Couldn't find the tour operator so I went out on my own. Get out for early am rides to beat the heat and the traffic. Some of the best restraunts in the world are there.
My schedule was: ride-eat-suntan-drink-eat-sleep-repeat.
One of my in laws is in Toronto going to school. He is one of the local journalists in Antigua and he helped with getting some of the cycling there some press.
He mentioned that road cycling is big there like mtb is big in Jamaica. But as he pointed out it's the easiest form of cycling to get into. But he says it will happen eventually. I hope to head down there in 2 years.