I've had one for almost three years now, and I love it. My praise is the usual: great design, easy to use, convenient as hell. I'd strongly recommend that you go into an Apple store, if possible, and play around with the different models. Mine is 15GB, which is quite enough. I don't have the need to carry around all 40GB of my music collection, and I don't have the time to wade through everything while I'm commuting. Negatives: the primary negative issue is battery life. I don't know about the latest models or the mini, but the battery on my model lasts about two years and then dies. Because of the construction, you have to send the entire unit back to Apple (or to a third party) for battery replacement. It costs about $90, plus shipping and insurance. Check out the iPod Support Discussions
for more on this. Secondary issues might include where to carry it, which headphones to use (the stock pair are crap), and how not to get mugged while using yours (once lost or stolen, they're nearly impossible to trace). But the negatives don't come close to overshadowing the positives, and should mine ever completely die I would replace it the very same day.