This actually seems like a pretty good deal if you have a lot of music that you not only want to get backed up, but also that you might want to listen to wen you're away from your library. The premium plan that costs around $40 per year is still available, and there do seem to be a few unique differences between the 2 plans that should be paid attention to:
- iTunes DRM -- the premium plan will back up, but no play your iTunes DRM music; the free plan makes no mention of the DRM files, so I'm guess that you may not be able to back those up. If you don't have a lot of DRM files, this probably is not an issue for you.
- Max file size -- the free plan will only store music 10MB or less per file while the premium plan stores music files of 50MB or less. You probably need to go through your library and see how this would impact you; it doesn't impact me at all because I don't have any files over 10MB.
- Album art viewing -- not sure why you would need this, but the free plan doesn't support it.
- Advertising -- although it's not mentioned in the bulleted list, it is mentioned on the sign-up page that the premium plan has "no ads." For those who aren't following along, the business model for the free plan is that it will be ad-supported.
- Playing music on web-capable devices -- this is only supported in the premium plan and will probably become a more relevant feature as more and more people get high-speed capable phones and data plans (think EvDo revA and HSDPA).
If you don't back up your music library at all, then MP3Tunes is probably very worth looking at.