John Gruber has used his Magic 8-Ball to answer your questions regarding the 'Napster To Go' subscription service. A must read before you fall for the trap.
I bought my iPod almost a year ago, and since then, I haven't bought a single song on iTunes or any other online music store. And not just because the iTMS was open for business in France only recently. My iTunes library is simply full with my CD collection, which my iPod helped me rediscover. Being a lover of baroque music, I find the catalog and sampling quality of the iTMS quite mediocre, and not a bargain compared to CDs from excellent (and independent, not a coincidence if you trust me) labels such as Harmonia Mundi or Ligia.
In my own perspective, Napster has absolutely nothing to offer. I completely resent the idea of renting music, and their monetary comparison with respect to the iPod is really stupid, but your mileage may vary. I'm listening on my iPod to CDs I bought more than a decade ago. Will Napster stand a decade? I really don't think so.