I have THREE TIMES missed connections at US hubs specifically because of the dire immigration queues. And in theory, all of those connections offered generous connecting time (around 3 hours). I don't care that the process will take just as long at MAN, my arrival time at the airport is under my control there. I do mind constantly missing advertised connections stateside and having to join a huge queue for re-booking. THAT is the beauty of pre-clearance. And yes, it is well worth having.
MAN also has the advantage of potentially introducing this in parallel with their major terminals redevelopment. They have the opportunity to design-in the best facility of its kind. Let's hope they do.