My house has two flights of stairs, wife kept nagging me to get a cordless (although I am the one that does the vacuuming). We ended up with a Dyson V10. Only cordless vacuum I have used but dont feel the need to try anything else. Work's impressively well and came with a lot of attachments (carpet + hard floor head too).
The battery lasts great but I can't do a 'deep clean' of my house in one charge. I can vacuum all the floors no probs which is the main thing, but if you are a bit OCD and like to vacuum skirting, door frames, window ledges etc then you might need a charge in between. For me it is not an issue as I can 'deep clean' one floor a week.