ill give you a short and a long version
short: buy finis swimp3, its awesome
long:
i have tried the first gen finis swimp3, the speedo lzr mp3 player with earbuds and earhooks and the new swimp3 g2
the swimp3
sound is clear and good (bass is not perfect, but gets really good if you wear earplugs while swimming)
keeps your ears clear, so you can hear what happens around you (nothing like being able to talk to strangers at the breaks and not having to dismount the mp3 system)
fits like a glove: are not in the way at all, feels natural when it sits in front of ear
buttons are easy to find: the operation of the buttons is so simple that you cannot go wrong, they sit so you can easily operate them doing flipturns
shuffle buttom and folders: you can mix the music you hear during warm up, and then easily switch to the more fastpaced for the intervals
speedo mp3 player with earhooks
when you wear a swimcap the hooks stay in the ear: which gives a better bass on the music, but i hate wearing a swimcap
the earbuds falls out doing flipturns.
buttons are easy to operate all the time
no shuffle buttons: always the same songs in the same row, it drives you nuts
if you still are in doubt, then here comes my story of how i tested if i needed a swimp3 and how i use it
started with the swimp3 in 2008, it was the 1.st edition, but died in 2010, it lacked that it only held battery for 6 hours and it had to be at full volume for the music to be easy to hear while swimming butterfly
my swim pusher switched it to a speedo lzr mp3 when the swimp3 died, which was a sad day. i now had to wear a swimcap to make sure the earbuds not fell out, no more shuffle button, buttons wasnt as easy to find and press… and it landed in my box of “stuff that i should sell when i find time for it” its still lying there…
so 6 months ago i started flirting with the idea of getting a swimp3 again… but i wasnt sure that it was nesecary? could i keep focus on swimming while music in the background? would it interfere with my technique? wasnt the 910xt enough electronic to put in a swimming pool? was i as a man able to multitask?
with this in mind i started testing the effect of focusing on swimming or on homework. the variables was the time my 500m warmup took and the times i got on a 10x50m interval swimming and the swolf score from the garmin. and finally a mentally score of how many times did i think “am i there yet” during the workout
i made a baseline wearing the watch and focusing on my stroke, then where i brought homework to the pool and started solving equations while swimming, and borrowed a friends swimp3 and had music playing while doing the sets. the music matched the style, slow and comfy for the warmup and hard techno for the intervals
my findings were:
thinking of homework compared to focus on swimming; my intervals got slower and got a higher swolf score (higher strokecount), but the warmup felt more relaxing since i was doing something else than swimming in my mind… so good idea but not while doing intervals
listening to music vs focusing: everything got faster and a lower swolf score while listening to music, i even got a couple of adrenaline rushes doing the intervals with music (music moves the mind) and the warmup felt nice and comfy, since i was almost singing along to my favourite country music 
but i properly looked like a retard while
listening to music vs homework: music was better, espically doing the intervals
so after this i went and bought the finis swimp3 it costed around 100$ and i was happy 
being able to start it on shuffle doing the warm up and easy sets listening to different music all the time is awesome, and then when its time for a serius race face, you switch to next folder, by simply pressing the next and power button once, and now you can listen to the dance and techno music that simply tells you “GO FASTER” and when done, you switch back to the easy music and enjoy the rest of the workout… the battery lasts 8 hours on full volume, but i have it on half volume which is more than enough where it lasts around 12 hours of music.
so go buy the swimp3 and be happy 