N800 looks good but...
...what about capacity? is 16GB enough?
Also, the cowon Q5 has a slightly bigger screen at 5inches. From using an Archos 604 (4.3"), I would say that the 5" is worth having over the 4.1". The price is important but once you've bought two 4GB SD cards, you will have spent over £300 for a sim free N800.
I have a problem with the N800 in that it is too big to use as a phone, but is perhaps not quite big enough to compare with the latest PMP's.
The open source aspect is enviable though, and it does have many desirable features.
Like I say, the problem is that I wouldn't use it as a phone, so since I'd use two devices anyway I might as well get the K850i cybershot (fits in my jean pocket) for the pub photos and the Cowon Q5 for the airplane/train rides.