What are people's views on winter tyres in summer? My neighbor has run winter tyres on her Polo all year. Do they turn to jelly above 7c?
Nope, not jelly, but of course you've got to use your noggin and just be aware you're best not hammering them when it's hot since the compound is softer than a summer tyre. Grip whether hot or cold is progressive anyway so they're quite predictable.
On my 2003 Focus wear levels are also ok again as long as you're fairly considerate in the way you drive. I did 37k miles on my last set with no issues. That distance covered 3 winters (if memory serves 1 was pretty bad, the second was better and the last winter was very mild) the only attention they needed was swapping front to back if the fronts were more worn than the rears. Ice and snow performance was understandable exemplary but I also found wet performance to be excellent too. Especially when running through deeper wet roads and standing water.
TBH I chose to run them year round initially as an experiment to simply see how they compared. Besides that it also meant I didn't have extra costs like buying additional summer tyres and matching rims plus the inconvenience/expense of storing them.
Another factor I began to consider whilst using them was one that Clarkey highlights in his post above. I discovered I'd rather be on winter tyres in the summer than summer tyres in the winter. It's a much more predictable and assured performance averaged across the changing seasons of the year.
My only final point (and one that I've made before) is that on larger cars I would say wear is naturally increased and I can only imagine the perceived summer performance may also decrease. It likely varies with cars though so it's very difficult to absolutely rule out a car just because it's large.
For example: A friend of mine had a 2003 Mondeo 2.0l TDCI which absolutely ate his winter tyres over a couple of winters (no summer driving) and yet his new 2010 Avensus estate has been on winter Nokians all year and delivered really great wear levels. His father also put similar on his Skoda Octavia Scout and with year round use managed around 35k miles.