I'm seriously thinking of paying for a subscription to Chinesepod - any opinions here on whether it's worth it? It seems to have a pretty good reputation among Chinese language learners..

Well, look at it this way: name another website that does what they're doing, has a forum with an active dialogue between users and the folks behind the scenes, and puts out their main product for free. Have you done their free trial? There's also apparently a 60-day guarantee.
And lest you think this is a shill...I'd try using them a whole lot more but, surprisingly in China, I've found there's only a narrow window during the day when the site isn't horrendously slow...at least with my ISP & location.

Apparently I can download all the mp3s for free (though I could be mistaken about that). I take it the premium stuff is worth the money?

You can get all the newbie audio lessons for free through iTunes but the higher levels, plus transcripts/translation PDFs etc., are paid-for content. Compared with dull CD-rom lessons, or a 80RMB/hour tutor who rushes through a textbook from the early 90s, it's def worth the money to get access...

Just my two cents, but I find myself using Popup Chinese almost exclusively these days. Their dialogues are faster and more challenging, but also more natural and their hosts don't speak in the irritating Shanghai dialect. I actually find it difficult to go back now.
Also, they're much less expensive.