Only worth it for one Pokemon you might not have yet in your collection. Mew.
Now that that's out of the way, let's get on to why it doesn't suck.
If you're working towards a legitimate Pokemon collection (this means no GTS negotiations or trading for Pokemon you could get in-game normally), then the Ranch is the only place you're going to get Mew. Outside of an event, of course.
The only problem? You better have Diamond and Pearl only. I own Platinum, and I had to borrow someone's Diamond game to get the Mew that I needed for my collection. If you live in Japan, Platinum will connect to the game, but nothing else will. They were supposed to release an update, but I guess they forgot about the ranch. It's not hard to do.
The Ranch boasts cuteness overload, an annoying main character, and free space for up to 1,000 Pokemon. Think that's a lot of room and how cool that is? Think again, you're going to need to fill up every space to get that Mew. That means you need to put in the equivalent of two full collections to get it filled up with Pokemon, in order to trade in-game for the Mew.
So if you're prepared to catch 500 or more Unown in order to get a Mew, go for this "game".