I've been syncing the pdfs attached to my Zotero records through the WebDAV service (via iDisk) for a while now, having previously grown suspicious of syncing the Zotero storage folder through Dropbox (particularly the sqlite database, which always seemed to have conflicts arising on different computers). The option of having them all in one place on Zotero's servers (bibliography and attached files) is a great addition to the product, and it seems to me that there are no bandwidth issues (so far) and everything syncs quite nicely. I'm using about 3.5gb of space right now, but could easily see myself reaching 10gb in the future. I assume they'll open it up further, as Dropbox and others have done, and it doesn't seem to me that $100 a year is too much to pay, particularly if it helps development of the app. For now, though, it's a very useful option for a great piece of (free) software. If you're still using EndNote for academic references, you need to run (not walk) to zotero.org and start importing your database.
To see the value of this syncing, take a look at my "items" list. This cataloging functionality is available for free. But what I noticed today is that, when logged into the Zotero site, the pdfs (if attached to the record) now appear at the bottom of the entry, which means I can always find a copy of a particular stored pdf no matter where I'm at, assuming I have web access. And, of course, in the absence of an official iPhone app for Zotero, this makes the site way more useful through Mobile Safari than it was before. (Incidentally, this is true for pdfs I've scanned in addition to things downloaded from JSTOR and the like. For visitors to my items list, the personally scanned pdfs are not available but links to JSTOR pdfs are, e.g. this item.)
For more info on Zotero file storage and syncing, check out the FAQ.