In reply to cliff shasby:
It works well as long as you get to Aosta early enough to get the last bus up to Cogne (can't remember if that one is direct or if you have to change).
The cost is certainly a lot less than hiring a car for a week if there's only one or two of you going. The expensive bit is the transfer to Cham - about €30-35 one way. You can go by Easybus, which I think is about half that price. Check where it stops in Cham but I imagine it's fairly central. The coach to Courmayeur is €15 one way and goes from Cham Sud (5 minute walk from the train station). From memory from a few years ago, the coach to Aosta is about €10 one way and up to Cogne is only about €5 one way. That's the one you don't want to miss. So you can do it for about €130 return per person, may be a bit less if you get Easybus or a promotional deal on the transfer to Cham.
Allow plenty of time for the journey from Courmayeur back to Aosta - it's about 45 mins per the timetable and on a clear run it will be spot on but sometimes you get random queues going from Italy to France (happens the other way as well but doesn't seem to be as often).
You might know this already but for the benefit of anyone else...
Couple of things to bear in mind if hiring from the French side of the airport:
1. The cars there don't come with winter tyres, so you'd need to hire chains and they don't give you particularly good ones. Really wouldn't recommend risking it without winter tyres or chains, for personal experience chains proved invaluable the one time I took a French hire care without winter tyres and it wasn't even snowing that much. The chains were a right pain to put on as they don't give you 'easy fit' adjustable ones but without them we'd have been unable to get around.
2. The cars there don't come with a Swiss motorway vignette, so that'll be 40 franks / euros unless you skilfully avoid taking the motorway on your way back to the airport. You really don't get a choice if you end up on the motorway into Geneva!
MB tunnel is about €55 return (make sure you get return as a single is about €45).
If you're staying in accommodation at one of the valley heads, you will be able to walk to the icefalls in that valley but getting to the other valley will be a bit of a mission. Not really an issue if there's loads to do but if you're going early season or if it's a poor season generally, you'd want to maximise your options. If you're staying in Cogne, there are local buses I think or you could try hitching lifts but it's a decent walk back if you miss the last bus back to Cogne. Not the end of the world and it would be on a road so at least there won't be lots of snow to wad through (unlike up in the valleys!) but not what you want at the end of a long day.
