In reply to anthonyh:
Very rare occasions the roads are completely close in Scotland, however it will be virtually impossible to do any winter climbing in Scotland without a car unless you are staying in CIC hut or something like it, in which case you will be force to stay there even if the conditions aren't good, it happens often that you have good conditions in the east (ie. Cairngorms) and bad in the west (ie. Ben Nevis), even within a week.
Mobility and capacity of changing plans is main key if you want to succeed
in Scotland, so in my opinion you need a car.
Enterprise in Edinburgh has good offers if book in advance on-line and the airport bus drop you by the door in 15min from the airport, £6 return I think.
I hope this helps.
Some of the smaller roads can be a problem but places like Glencoe, the Ben and the Northern Corries are normally accessible. Things to watch out for are the high passes on the A9 and A82/Rannoch Moor at night in bad weather as you can get stuck up there if the ploughs aren't coping.
Might be worth checking if your car hire company kit the car out with snow socks. They do this as standard in Norway but no idea if the Scottish companies have cottoned on to this.
In reply to anthonyh: I would also advise taking a sleeping bag even if you don't need one cos of your accommodation. I have had to kip in the car overnight cos of the snow/roads once every few years up here.