The Dension works well but as has been said it's expensive. I installed mine myself, it isn't that difficult, all you're really doing is unlatching a couple of fiber connectors to jump it in to the loop. While there's probably no substitute for having a pro do it you can do it yourself in an hour or two. I did a bit of a write-up on my install here
http://www.radarrack.com/boxster.html happy to answer questions if anyone has any. If you have PCM the interface from the head unit is good enough that you can install it in the front trunk and put the ipod in there, if you have a CDR23 (and I'd guess the CDR24 is the same) the control interface is poor so you'd want the ipod in the car with you so you can use it to access music.
Chris.