To help kick things off,
Before you go thinking all this glorious music is free, let me fill you in on the pricing. You get a free 2-month subscription to the music store when you buy a SanDisk Sansa, which will set you back $139 for the 2gig model and $249 for the 8gig model. The subscription is for unlimited tunes. The catch? If you don't sign up for a not-free subscription after 2 months, you won't be able to listen to those songs anymore. The basic subscription service will normally be priced at $14.99/month but you can also just "buy" individual tracks for 99 cents each, just like iTunes.
I don't know about you, but I prefer my digital music services and devices to have names I can pronounce and easy acronyms for when I'm feeling particularly lazy, so BBDMS + Rhapsody 4.0 + SDSe200RRMP3(p) = headache, whereas Apple + iTS + iPod = the true Best Buy.