I ran a Fiat Marea Weekend 130JTD (Estate with the 5 cylinder diesel) for 15 months continuous on Goodyear Ultragrip 8 winter tyres. I covered about 25,000 miles and took 2mm off the fronts, and 1mm off the rear. To say I was astonished at the lack of wear would be a serious understatement since my previous car chomped through its OEM Continental Sportcontact 3s in only 8,000 miles for the fronts.
During the summer months I did not notice any difference in handling to summer tyres. I think you would have to push beyond 9/10's to notice. In winter on cold roads I had more confidence, and when it finally came to snow and ice on the roads the difference was night and day. Pushing a £40k Audi A5 up a hill out of the Metro Centre in my £1000 Fiat had me in stitches.
Anyway, as for all seasons. The phrase 'Jack of all trades, master of none' springs to mind. This video from tyrerack explains it best (skip 30 seconds in)
http://www.youtube.com/watch?v=L2wTg0l3_wI