In reply to TomAlford:
Alpine climbing covers many route types, grades and approaches. From the fast and light ( or slow and light in my case ) to being fully laden for a bivvy or multi day routes. Routes / terrain I feel comfortable on will often mean I only carry 3-8 wires and some slings, other routes require a larger rack. So, in my opinion, there is no one rucksack that does it all.
You will have a much better idea of what you might want / need after your first season - could you borrow a sack so you can buy from a more personally informed position?
People have made great recommendations and they are all worth listening to, but bear in mind that most people will like what they have bought because it is what they are used to and they bought it ( presumably ) for their preferred types of alpine climbing. I would also echo very strongly the comments about finding the rucksack shape / size / back / straps that are comfortable for your personal body shape. Try sacks on with as many ropes or other heavy gear as you can stuff into it.
I would recommend turning the question around from 'what rucksack should I buy?' to 'what features do I want from the rucksack' and then look for a sack that most closely fits the bill. As a part of this, gather what gear you would take on an alpine route ( including any current bivvy gear you would use, remember food and water ) and put it into different sizes of rucksack to see what size sack you currently need. Remember, as you gain experience you may find you can use smaller / lighter rucksack, sleeping mat, stove, ropes, technical equipment etc..
By doing this I have ended up using two different sacks for the climbing I do:
1) A very light 25l sack for day routes or when I don't need to carry much. Minimal features, basic waist strap that doesn't get in the way of harness, very comfortable slim and stable for use on more technical rock climbs, quick and easy access whilst at climbing stances. Can stow rope / axes easily. Single pocket for photocopy of guidebook / route. Also can fold away into a small unit to be carried in my second pack if appropriate. Currently I use a Patagonia 25l Ascensionist for this. However, compromise is that it wouldn't last long if I used it in too many Chamonix chimneys!
2) A larger more robust 35 - 45/50 litre sack that can hold all the technical gear plus bivvy gear / food for longer / harder routes or where I choose to bivvy instead of using huts . This needs to feel comfortable when fully loaded so requires broader waist straps and various tension straps. I prefer a floating lid for greater versatility. Even better is a sack that can then be stripped down. For example removing the lid, removing the broad waist strap leaving a basic one in its place, removing back plate / support so you end up with a lighter ( albeit shapeless ) bag that weighs around 600 - 900 grammes. Many so called alpine packs, in my opinion, have more features than you need and weigh well over a kilo. Of course, you can also cut stuff off! At the moment I have one of the old school Pod Black Ice for this.
So, in conclusion, go try lots of sacks from as many mates and in as many different shops as possible. If you can wait, consider the European shops in the alpine areas you are going to - often lots more variety. And don't forget to match the colour with the rest of your kit for that experienced and stylish alpine climber look in your photos!
Sorry about lengthy reply, sitting out a storm in Chamonix with nothing better to do! Enjoy your first season...