If you're willing to shell out another $100 for a set of bluetooth headphones: it's great.
Otherwise: it sucks in oh-so-many ways.
-The sound quality of headphones that come with the music kit is abyssmal.
-The cord for them is very short: I could barely stand upright on a cardio machine without pulling the phone from the control panel (I'm 5'11").
-The port is on the side of the phone, not the top, which is more irritating than you might imagine.
-The port is not a standard stereo plug: it's a propietary design so you need to use an adapter to use anything other than the terrible provided headphones. As if that weren't enough, the adapter is for a 2.5mm plug, so to use most decent headphones you'll need an 3.5mm adapter for your adapter: the whole contraption is incredibly clunky and has a tendancy towards poor connection.
-Since the phone locks itself after a few seconds, you'll have to unlock it every time you want to skip a song or adjust the volume. To do that, you have to press the "Unlock" button on one side of the phone (twice) without pressing the symetrically placed "End" button on the other. It's hard enought to do in the calmest moments, never mind while running. Not a really well thought out bit of design there, that.
That said, I did shell out another hundred bucks for a pair of Motorola HT820 bluetooth headphones.
Now my gym experience is almost seamless. Other than looking like Shrek (maybe not an issue if you don't shave your head) and occassionally redialing my last call by accident (because I'm an idiot), I have no problems with it at all.
Other than that night in Thailand, it's the best $100 I ever spent.