Spirit Tracks is definitely a good game, but honestly it doesn't have enough replay value to warrant a buy. On the list of games to rent, though, ST is a shoo-in. The main issue is that you'll never find some secret area you didn't see before, like in better Zeldas. Everything in the world is right on the side of the tracks, and the only thing close to exploration is choosing whether or not to turn left at the tracks that aren't going straight toward your destination. Instead of discovery, it's more of an issue of patience to discover new things.
Otherwise, though, it's a great improvement on PH's controls and engine. The dungeons are just fantastic (my personal favorite being the stellar Sand Temple), and the Tower of Spirits is really pretty tricky in places- going without a guide is honestly a real challenge. The Phantom works great, too, and the dynamics with Cole, Byrne, and Anjean are pretty deep near the end. I'm pretty peeved about Malladus, though- honestly, the game would have been much more interesting if we had that creepy evil leprechaun directly trying to take over the world. Cole was cool, but Malladus was, um... lessee... dumb. -adus. He only had like three lines and they were all just like "argh how dare you" type stuff. Plus his whole MO was a complete ripoff of Ganon, despite him being a new villain. The new items work great, and some minigames are genuinely challenging (scaling the giant tree in Whittleton, the third Take 'em all on level, etc.
It's a good game, but again there's very little replay value. Definitely rent.