As I have said before, I have never nor would never use them especially after being given advise not to do so from two professional plumbers.
I must say also I love plumbing with copper and forming the joints, so a Lego type system that can "pop" at anytime due to pressure fluctuations and age does not impress me. The effects on electrical earthing is another factor that puts me off.
Why give yourself a chance of collapsing ceilings and damaged walls when using push-fits when you could do everything very securely with standard copper fittings?
I am with you, Lizzie.
Plumbing with copper pipe is one of those skills that is just satisfying to practice every now and then. There may not be any reason for it these days, but, a bit like driving a proper car with 3 pedals, I find it satisfying to do it well and to do so requires occasional practice.
I have used plastic plumbing for convenience in the past, but only in temporary or "under the sink" situations where a leak is immediately obvious and not too damaging. I wouldn't for a minute use it on central heating systems, under floors, and anywhere inaccessible, but I've come across plenty of installations where this has been done, and by real plumbers too.
As said, give it 20 years and the rubber O rings will perish and you'll be in all sorts of bother, especially in a central heating system where the water can get very hot and be subject to frequent temperature fluctuations.