Let me start by saying that when I first heard of it, I thought it was "gimicky" and wasn't too sure. I got one of these, based soley on the enthusiasm of the guys in my local shop. I've used it on every dive since I've owned it. I love it.
If you hang the speakers from the mask strap as is mentioned above, the sound is reasonable. If you take the "hangers" off and wedge the speakers under the mask strap or your hood, so the press against your temple, the sound is much, much better. (Bone conduction??)
Task loading is minimal, and all of the normal MP3 player functions on the Oceanic/iRiver can be accessed underwater. (Shuffle mode, volume, track skip etc etc)
I have older Pink Floyd loaded into mine. Darkside of the Moon, Echos, Wish You Were Here. That type of thing. My girlfriend has Metallica. I find I breath a lot heavier if I listen to something heavy like that, but the long slow strains in most of PF's music is great for me. Very surreal to do a night drift dive with DSotM playing as you drift over the reef under the moonlight!!

And for something like a Sharktooth dive in Venice, where you're just hovering over the bottom looking for Megalodon teeth, it changes the experience of the entire dive....
Saftey stops??? They just disappear!!! There is something to turn your mind to as you hover in the water column for what sometimes seems forever. I've even stayed longer, to complete a song.

How can that be a bad thing?!?
You just want to remember to turn it off before you do the final ascent. It helps to hear the boat traffic above you.
I get the "WHY use one?" question from time to time on the boat and it always seems like an odd question. Almost everyone likes music and has a few albums or songs that they just REALLY like. Everyone on a dive boat likes to dive. Seems logical to me to combine two things that you enjoy to add something to both events. In this case, it even makes a bit more sense, since both events are "private matters" anyway, if you see my point.
My recomendation to people so far has been: If you have an MP3 player for any other activity you do on land, you'll love having one to dive with. If you don't use one on land, I guess you won't see the point of one in the water.
After trying to explain, I still have had a couple people look at me and roll their eyes like I was somehow going to destroy their dive by contaminating the water. But like I said, my buddy and I dive side by side, and have radically different choices. The music doesn't "bleed over" and we both really enjoy them.
If you're thinking about getting one, I would recommend it!