I thought my Zen Xtra was awful as well. First, the software for transferring was terrible. I did buy notmad, which made it much better, but that is totally unacceptable that it requires me to buy 3rd party software so the product works properly. Next, the build quality isn't all that great. If you've every held the ipod, it feels solid and sturdy. Not the Xtra. THe Xtra is ligher and flimsy, the front sags under pressure from your thumb and worst of all the front panel always pops out because the catch at the bottom was too loose. The user interface was terrible - it was very slow, got faster after a firmware update, but still lagged behind, as well as taking a long time, not to mention a frustrating time, to find any song or browse your collection. The thumb dial thing on the side is frustrating because it is small and it is a rocker dial - you have to push down tonnes of times to scroll a bunch. It would have been better for a scrolly dial instead. The unit is a fair bit larger than the ipod, has no remote control, and has a pretty poor case design (no window, cant plug the charger in with the case closed, button that closes the case was difficult to snap and unsnap). Last of all, my unit had a intermittant problem; every so often one channel would start distorting and clip the music - the unit had to be switched off for 2 minutes or reset to fix it. And my headphone jack broke - my fault but shouldn't have broken so easily.
As for pros, it was cheap when it first came out in relation to the ipod. Now that the ipod has reduced prices, the allure of the zen is simply not there.