I've got a Victorinox pack with wheels.
It's this one but I think mine is 55L. http://www.ebags.co.uk/victorinox_swiss_army/trek_pack_plus_3_0_56cm_wheeled_backpack_with_docking_daypack/product_detail/index.cfm?modelid=83927
I got it from Ebay so saved about 40%.
I only did a 2 week trip with it but I absolutely loved having the ability to carry it on my back or just pull it along. I had it really full and so it was heavy, but yet felt very comfortable on my back going up and the the stairs in the tube stations and the size is definitely the perfect size for a small framed girl like me. I really felt sorry for my mate as he had a really large pack and had to keep it on his back when walking through the airport whilst I smoothly rolled mine!! And also it was really difficult to move his bag along the floor when we were queueing.
Yay to hybrid packs from me!!