I have a 20GB iRiver, and love it dearly. Like half_of_monty's Samsung, there's no need for specialised software to take things on and off - you just use standard file-copying commands. As well as simplicity, this has the advantage that they can't tie you down with evil Digital Rights Management (spit!) restrictions. The interface isn't quite as shiny as an iPod (no clickwheel, but I think they have something similar on later models). Claimed battery life is 16 hours - I don't think it's that good in practice, but you may remember that I left it running overnight once and still had half the battery left in the morning. I see that this place are selling the 20GB model for £139 - you might be able to do better elsewhere, or on eBay.

As for capacity, big is definitely good, for all the reasons that neoanjou says. Frankly, I wish I had the 40GB model.