It's like Mii Channel, but for your Pokemon.
The first time you play the game (sorry, I mean play the "box") you'll meet Hayley, the ranch owner, and six of her Pokemon. She'll then ask you to help her expand her ranch by uploading Pokemon from your Diamond or Pearl game. This brings me to an important point: DO NOT get this game unless you already have Diamond or Pearl. Even with Diamond and Pearl, there's not a lot to do in the ranch, so if you're not even going to upload your own Pokemon then don't even bother with this title. It's essentially an expansion pack for Diamond and Pearl that just happens to be on another system.
At first, you can't really do anything in your ranch. At all. Take pictures maybe, but other than that it's just watching your Miis and Pokemon walk around and play with each other. Once you get an upgrade you can maneuver the camera and move characters around, but there's still very little player-to-character interaction, so it still feels like you're just watching them do whatever they please. Sure, there are some interesting interactions you can watch (like when Pichu rides on Pikachu's head while another Pichu gleefully strides alongside), but they're not exactly worth the $10 price of admission.
There are some redeeming qualities, however, that Diamond and Pearl owners can find useful. Most notably, Hayley will ask you to upload Pokemon that you haven't got in your Pokedex yet, as a sort of encouragement to catch them. She'll also tell you exactly how to obtain that Pokemon, and give you a 2-week deadline to catch it. These tips aren't exactly necessary in a world where all of our gaming questions can be answered by the all-knowing internet, but for players who are feeling overwhelmed trying to get their 'dex finished this feature should give them a push in the right direction.
Hayley will also sometimes want to trade Pokemon with you. While they're not always the greatest trades ever (I traded a Dustox for a Staravia yesterday), her Pokemon hold rare Berries that are hard to find otherwise. Also, I heard there's a Mew in it for you if you fill the entire ranch with 1000 Pokemon (although that's the Japanese version, who knows if they changed it for the American version).
Graphics-wise, the Pokemon models in this game make Pokemon Stadium for N64 look like a masterpiece. All of the Pokemon are small and blocky, and while they're still recognizable, some of them are pretty disfigured. Although it's overly simplistic, I have to say it is quite adorable (I actually said "aw, cute!" out loud when Hayley's Pokemon made their first entrance). It works on a few levels, but I wish they had put a little more into the graphics on this one. Some Pokemon aren't supposed to be cute.
Overall, this game gets by on its function more than its form. It's gameplay is shallow and mostly uninteractive, and it's graphics are overly simplistic. It's really here to perform a service to Pokemon players rather than entertainment. If you need some direction with your Pokedex or some extra space for your Pokemon collection, then this is probably worth looking into. Otherwise, you won't find much here.