Metal sleepers were around in B.R. days but were not not liked and were used very sparingly and not usually replaced with the same. The privatised railway has taken them on in a big way, no doubt because they are cheaper and they can cover larger areas of relayed railway a lot quicker. But whether they are more economical than the standard concrete sleepers, in the long run, is a debatable issue that only time will tell. You probably travel over many miles of them these days, not only on freight lines but once the ballast is layed you don't always realise you are running over these type of sleeper unless you really look for them.
Perhaps a more modern design and metal mix may improve them compared to previous makes but that would only be a guess.
The Settle & Carlisle line has steel sleepers laid on a very vast majority of the route and is going to be closed for a lot of this year due to track bed renewal, as the ballast is the wrong stuff!
If anyone's planning on traveling over the S&C this year, I would advise them to check the possession dates in advance to avoid disappointment.