I have a Rio Karma, which is an oddly-shaped 20gb player. It has great sound quality, features, battery life and UI. (I've never liked the iPod UI, as it requires you to go up and down through every level of the menu system. It's so over-simplified that it detracts from my user experience. *That's* what I would call a tortured UI.) I've never used the Carbon, but I've read great things about it. To me, though, its big downside is the lack of on-the-fly playlisting. That is, you can only play songs through the standard menu system (artist, album, track, etc) or through a playlist that you generated on your computer and transferred over. Perhaps it's not such an issue on a 5gb player, but this is one of my most-used features on the Karma (and it's something that the Karma does particularly well, in my opinion). From my search for a 5gb player, the only one I have found with on-the-fly playlisting is the Zen Micro. I haven't been able to find the Micro in stores, so I couldn't give you and opinion on it other than to say that the specs look pretty darn good (minus the battery life, of course).
I would recommend that you check out
www.riovolution.com, which has great user forums. There are a number of threads in the Nitrus/Carbon section comparing the various "mini" players.