IPods won't play .wma which is really stupid IMO. They also are all rechargeable, which stinks if you are away from a power source for an extended length of time. I like good old fashioned disposable batteries myself. But the biggest benefit to owning an IPod is that they have all the accessories you could ever want. I have an IRiver which is perfect in and of itself, but the inability to get any accessories that are any good really sucks.
I may switch to a Zen if they offer more in the way of that.