Yeah, the Napster service is utterly useless.
As far as I can tell the rental model only allows you to play your 'rented' files in media player and a (very) few selected portable players. I tried a couple of weeks ago; winamp and a couple of other media players wouldn't touch the napster files.
If you want do do anything with your files (e.g. burn to CD, convert to another format) then you need to buy that track (.99 each). Even then, I've found that many programs choke - e.g. Nero would burn OK, but if you want to do advanced mixing the audio editor would tend to hang. I think, but didn't verify, that even after you've bought a track you only get a limited number of burns or conversions before the file locks down completely - I may be wrong on this.
In short, the monthly fee seems very steep for what you actually get from it. If their files were more useful, then I could cope with buying tracks individually (like iTunes), but not in their current format.
My free trial lasted about 48 hours, before I decided it was useless. I terminated even more quickly than I was going to because my credit card company actually rang me up to question the charge placed on my card, when I signed up for the free trial. Yup - Napster charged my card when I signed up (at least, that's what my bank told me), although I have now got the statement, and it appears the charge was cancelled.