I love the all-inclusive concept - and have been to about a half-dozen in Mexico and the Caribbean. If you like to eat and drink, and not have to worry about paying everytime you do so, it is well worth it.
Some places I have stayed at (or sent clients to) that were exceptional are:
1. Secrets Excellence - Punta Cana - excellent 6 star property, without the 6 star price tag
2. Sandals St. Lucia - a bit pricey, but great for honeymooners
3. Ladera, St. Lucia - not technically an all-incl prop, but they do offer a meal plan. Truly one of the most remarkable resorts in the world. The resort is set on a mountain, and all rooms only have 3 walls - the 4th wall is non-existent and faces the sea and the mountains! Go here for more info:
http://www.ladera.com/index.asp
4. Royal Hideaway Playacar, Riviera Maya, Mexico
If you like you can PM me for more info.