I have used the zen for about 1 hour and it crashed twice. I used the nomad 3 for days, and the creative software application was extremely buggy. It would often lose track of my mp3 player. Sometimes during the middle of a transfer. Sometimes it wouldn't recognize the player when I attached it. Finally, it would just sporadically crash.
As far as the iRiver goes, as I mentioned many times before, my experience with iRiver is that it usually sounds good but the software bugs cause some really weird and awful sound sometimes (in the case of the slimX, often). It also depends on your use. I use mp3 players in numerous environments... attached to my pc speakers, with etymotic earphones, with/without headphone amps, mx500's, tape adapters, line outs to my home theater, and so forth.
No player sounds good in all of those environments. Rarely will they sound good in even ONE environment compared to a cd. AAC on the iPod sounded generally better than the iRiver units I tested (one USB and a couple cd players), again due to the weird anomalies in iRiver playback. The zen sounded pretty good, but the user interface was just too weak to deal with.