Time for a progress report. I'm 20 hours deep, saved in the first room of the 4th dungeon.
Some advice: I view the dowsing mechanic largely as a crutch, a thing that somewhat discourages exploration. As a result, I don't really use it unless I'm very lost as to the next port of call, which has only happened once or twice.
The sequence of events leading up to the fourth dungeon is worth mentioning. The part in which you have to set things in motion again in Skyloft - the bit that Broodwars is apparently at - does have a few too many unneeded steps (like retrieving one of the windmills), but once you return to Faron Woods, the game gets back on track, as you are required to use new abilities in an early area to proceed.
I had my first Siren Realm experience, which given the apprehensions everybody had pre-release, was much more fun than I expected. If I may describe it in some detail:
The comparisons to the tear hunt in Twilight Princess and the Ocean King temple in Phantom Hourglass have been exaggerated to the extreme. Yeah, you're collecting tears and there are Guardians after you, but the dynamic is very different; in my opinion, it's more intense, while at the same time more manageable. Basically, they are always chasing you, but every time you collect a tear, the Guardians are disabled for 90 seconds, allowing you to look around with less tension. Plus, there are these Light Fruit pick-ups, which will temporarily place beacons on the tears. However, certain things in the environment will trigger an 'alarm', so to speak, cancelling that stuff out and putting the Guardians back on your tail. There's no denying that the Siren Realm is far better realised than the aforementioned sections of those other games, and I'm actually intrigued to see how it plays out in the other areas, which are quite different in geometry terms.
Later, you need to re-enter the 1st dungeon to collect an item. Fortunately, unlike a couple of the more time-consuming and less interesting "fetch quests" you've had to do on the way here, this was well done. For one, all the shortcuts are still in place, so cutting through this dungeon is quick and easy. Different enemies have also populated the place, which changes things up a little. Best of all, there are a couple of new puzzles that utilise items you've obtained since the last time you came here. I may have even found a Piece of Heart that you couldn't have gotten previously, although don't quote me on that -- I might have been silly the first time through.
Looking forward to seeing what dungeon #4 can bring to the table. Dungeon #3 set the bar high.