My experience with decent carpets is that Tredair type underlay expires
long before the carpet is worn out. Especially in heavily used areas.
I've just re-done the hall and stairs with Cloud 9 (same carpet) and it
seems great. Only time will tell if it lasts.

I was going to go with Cloud 9, but the vendors' site also had a
"what's best" page which suggests Tredaire Dreamwalk 11mm - it's a
foam just like Cloud 9, and sod all price difference, so we went with
that.

It seems pretty good so far (only laid a few weeks ago) - everyone
who's been to the house since has commented on how nice it is (i've
forgotten what carpet felt like so i'm not the best judge atm).