I use 8gb Class 6 cards on mine. I usually transfer/sync albums using Windows Media Player and the refresh has typically been roughly 5 minutes with 80% of the card now filled on both Clip Zip and completely filled on Clip+.
I'm constantly amazed hearing people waiting for half an hour on these refreshes. I don't think I'll have enough patience for that. I'd be too bored waiting on the songs. I say this because I occasionally have problems with card being bumped on Clip+ when I'm doing sports. The database would need to be refreshed every time this happened.
I now have the mentality of treating microSD cards like CD media. Swap the card out to the songs of your liking. You don't HAVE to stick to the same card every time!