We use a 40GB Jobo GigaOne all the time - stick a card in it, push copy button and then leave it to do its stuff. When the card's copied the GigaOne powers itself down, so you can just leave it and get on with other stuff (like more shooting).
It creates a new folder everytime you plug a card in, so there's no chance of overwriting images numbers from different cameras - and if you accidentally put the same card in twice, you get two copies of it in separate folders. Battery lasts a long time too - our son and his wife took ours to New Zealand for 3 weeks and never needed to recharge the battery while they were away.
Works really well for us, and allows us to back-up cards as we go during a wedding (so we've got 2 copies of every image before we leave the venue).
No idea if the other similar devices work this well, because we've never used them.
Rob