It's basically a $10 screen saver that is a huge cash in. As mentioned, Pokemon you deposit can only go back to the same cart and the same save file. If you start a new game on that cart, your Pokemon will be stuck on Ranch forever.
Very few people EVER need an extra 1,000 storage spaces. So there's a few circumstances this'll be a wise purchase:
1. You're a huge Pokemon fan and will purchase almost anything.
2. You're an obsessive breeder or collector of some sort and need the space.
3. Easy way to get Mew and Phione.
With just a few minor tweaks, such as wi-fi trading (other players can visit your ranch, that's it), or transfers between different carts, it'd be so much more appealing. There's almost no reason this couldn't have been on PBR, but hey, Nintendo knows they can sucker fans into purchasing it.
Buy it if you fall into one of the 3 categories. Otherwise, hope they come out with something better.