I've played several Harvest Moon games over the years, on both GBA and Nintendo DS.
This one is rather different from the earlier games in that it takes place on various islands rather in the traditional 'village'. Yet the basic premise of farming and socialising with your neighbours is very much the same. There are new characters (although most of them I believe were also in Islands of Happiness, which I haven't played).
Other games in the series have been criticised for having to use the stylus to move around, which is incredibly annoying for those of us that prefer using the control pad. Luckily, this is back to traditional control pad functions for movement and almost everything else, although there are various stylus alternatives for things if you prefer.
As you're used to if you've ever played a Harvest Moon game, you can acquire farm animals - such as cows, sheep and chickens - to care for, as well as pets such as a dog and a cat. Your main 'aim' (although this is very much up to you and how you choose to play) is to make a success of your farm, with animals, crops, etc etc. You can also spend time doing things such as befriending the other islanders, get married to one of the guys/girls if you build your relationship enough, go fishing, mining, learn to cook dozens of recipes, and much much more.
An added element of gameplay in Sunshine Islands is finding the sun stones, which enables you to discover more islands to explore and people to meet. There are 100 in total, so there's plenty to keep you busy, as many have tasks that you must complete before you can get hold of them.
I would thoroughly recommend this to anyone aged about 11+, who enjoys games that aren't just simple platform games with a quick ending and not much story. This is much more like games such as the Sims, and it can go on for as long as you want to play, with more things to find and improve at all the time. As with all Harvest Moon games, you can play this for literally hundreds and even thousands of hours, without ever reaching 'the end' and having to start over.