I've had my 16Gb Mozaic for close on 2 years now and it's still going strong.
Small enough to slip into any pocket, big enough to hold 4000+ tracks, it'll easily go a week between charges (based on my daily 3 hour total commute) even after 2 years... as an mp3 player I cannot fault it.
Tracks are a simple drag & drop (don't bother installing Creative's bundled software, it's crap!). The FM radio is a useful function. Simple, intuitive menus, selectable play orders (alphabetically, by artist, by album, random, etc)
Screen is really too small to use for watching videos or viewing photos, so I don't bother with either. Speaker is tinny (as well as socially obnoxious) so decent headphones (Skullcandy Ink'd in my case, replacing the cheap and nasty ones supplied) will let the sound quality shine through.
So pros:
Large storage
Small physical size
Great sound
Good battery life
Simple to use
Cons:
Software
Supplied headphones
Built-in speaker
Small screen