I have spent a lot of time using the lifts at various underground stations, mostly on the Jubilee line and they are very rarely not working and excellent if you have a buggy etc. Generally they are indoors which helps but one of the reasons I suspect the cost is so high is that they will be quite high spec. to minimise the potential down time.
The biggest problem will be people who don't 'need' to use the lifts using them all the time and those that do having to wait. Lets hope people are responsible and let the people who need them use them.