I woudln't say that battery life is a major problem with the Archos, it would probably do a movie and a bit, but it's certainly not infinite. If someone is in the habit of taking long plane journeys then a second battery might be a good idea, unless there is a means of in flight charging.
I was able to get a long life battery for my Archos Gmini 540. That's great as it's claimed to give around 13 hours video play life off one charge. It did however vastly bulk up the size and weight of the unit, whcih was barely pokcet sized in the first place.
Have a
for Archos accessories. There are quite a few suppliers out there of third part add ons. I've bought stuff in the past from newmp3technology.com and found their range quite good.