I love dowsing, especially for hearts. It has saved me way to many times. Also, dowsing is helpful when you are looking for key items, like the pieces of key that you need to open the Earth temple door. Dowsing always pointed me in the right direction. It not always led me straight forward to the thing that i want, so i didn't rely on it too much.(I learned to not rely too much by not using it when i do not need it.)
I loved the Dowsing ability, because I kept looking for the side quest items and gratitude crystals and goddess cubes. I would have never found all the items without the dowsing ability. It also reminds me of the dowsing item finder in Pokemon.
I Hated It At First, Because It Was Hard To Get Used To And I Couldn't Find Anything, But Then I Like It After I Got Used To It. Now That I've Beat The Game, It's Kinda Redundant And Doesn't Really Help Me Find Treasure Or The Goddess Cubes Very Well.
Well, now is probably not a good time for me to answer this question, considering I just completed the most grueling portion of the game thus far. That is,
trying to dowse to find the five shards of the Earth Temple key.
That was the most annoying, frustrating portion of a game I've experienced in a really long time. See, the dowsing is really nice... when you're only dealing with one level or floor. When there are different levels of elevation, dowsing is just incredibly inaccurate. Not to mention that it was incredibly difficult to dowse accurately due to the gigantic mountains I couldn't actually see around. Although, I have to admit, if it weren't for dowsing, I would have definitely cracked and started using a walkthrough to attempt to find the shards. It wasn't the dowsing that was frustrating, it was the actual locating and reaching the shards.
I don't use dowsing often, but because it has actually helped me (and prevented me from using a walkthrough like I always do), I'd probably say I love it. It's an interesting new mechanic, but one I think fits Skyward Sword nicely. Many aspects of SS are more technologically advanced, like the Beetle and Fi herself. It's only fitting that Fi, who sounds like an actual robot, could come up with something as techy as dowsing. I won't vote until I finish the game, but as it stands, dowsing is quite convenient and I'll probably be grateful for it later on.
I don't really like it, it doesn't have much use outside of the story line, and when you dowse for things like "Hearts" or "Gratitude crystals" it wont send you to places like grass and people who have them, only if they are physically there. (which truly, how often is that? almost never, at least not without you knowing.)