Working at a retailer that sells both, and from a cnet comparison, the CVM definately has a significantly better screen. The contrast and overall viewability is much better. Darker videos are difficult to see on the iPod, and the colors generally don't seem as vibrant.
You can adjust to any user interface if you use it enough. And don't forget the CVM supports some additional file formats, and has an FM radio.
Accessory-wise, the only place you will really be hurting is case selection. Car mounts, FM transmitters, chargers are all available, and there are plenty of cases, just not as many.
I'm looking at the ipod 5G 30GB, Gigabeat S, and Zen Vision: M.
I've never been heavy anti-ipod, but i'm really looking for something different. However, ipod seems to have the best options as far as accesories go. Also, I find the click wheel to be the best user interface.
I like the FM radio in the Gigabeat and ZVM, and the record option in the latter is a definite plus, but i'm not sure if it can balance out the sheer ease of use that ipod has.
Also, I have a LOT of music, and i prefer to not only have album/artist lists of music, but i want to create my own custom playlists. I'm not sure if the Gigabeat and ZVM can do that.