Finished the Ocean Temple (3rd one) and after a bit of exploring I'm about to do the 4th Tower of Spirits run. Tomorrow that is. It's 3 am here. =P
I'm enjoying it too, somewhat, though it feels rather short. There are portions of the game I wish would last longer, towns I wish had more to offer, and a noticeable lack of explorable areas--most of the exploring is by train, with very few large open areas traveled by foot. The soundtrack is decent (featuring several great themes, no doubt), but some tracks are a little iffy and the variety is still underwhelming.
As for dungeons, so far the Tower of Spirits seems more similar in structure, difficulty, and vibe to the classic Zelda temples (for me classic = ALttP), so I've been enjoying dungeon crawls through that one immensely, while the regular dungeons are short and simple for the most part. They are in fact marginally more challenging than Phantom Hourglass, though.
However I still have a few areas to cover, and from what some of the NPCs have mentioned to me/what I've read around the 'net it seems that a few towns I've already visited will indeed make purposeful returns, and I'm still anticipating the on-foot segments and dungeons to come.
It's hard to compare to Phantom Hourglass because despite their similar designs the two really are fundamentally different games, as odd as that may be. Spirit Tracks has an entirely new feel in comparison, but at this point I don't really know which I prefer. ST certainly has a liveliness to it that PH did not, but even now I'm already feeling a slight tinge of nostalgia for PH despite that it came out merely 2 years ago and I remember thinking it to be moderately good at most.
I liked Phantom Hourglass to a reasonable degree, and I'm liking Spirit Tracks very much too. More? It's hard to say at this point. The rest of the game may have more of an influence on my stance here.