I intend putting Winters on this year, I looked at the BMW sets and found them a little expensive .... the route I have gone down (which I have succesfully done in the past with my Volvos) is ........
I have bought a set of BMW 18" Alloys with summer tyres from a BMW main dealer (that were removed on delivery) via ebay for under £1k .... this gives me my next set of summer tyres and a spare set of alloys for the cost of a set of tyres, so the alloys end up at almost £0 cost.
Then just put the winter tyres of your choice on the new alloys ! .....
A bit of outlay but a very good saving overall !
Also I used these last year
http://www.konig.it/Products/Snow%20.../K-Summit.aspx ..... brilliant device, easy to fit and very effective, you can buy them badged as 'Thule' but it adds about £20 per pair.
Cheers
M