I finished reading the revised Gunslinger on the weekend. I'm usually not all that excited by people who revise their previous work at a later stage of life. It might be a 'director's cut', but the director isn't necessarily the same person as he was first time round (Star Wars is a good example). Also, stuff that was cut from the original might have been cut for a reason (I prefer the original, leaner The Stand, even if King says it was only an economic decision).
But this one was worth doing. Some of the sparseness of the original has been lost, filled with detail that makes the book fit better with the series, but not necessarily a stronger novel (which isn't really a sin). Some things have been clarified (I understood the whole point of David better--why he took Cort by surprise), some things simplified (I was a bit sad to see 'parsecs' go, but that was pure nostalgia), and it has been filled out well. People like Roland's mother seemed to be more strongly defined, yet the extra details didn't break the flow.
A couple of prominent additions appear right at the beginning. There's a quote from Thomas Wolfe which is an excellent choice, and the phrases '19' and 'Resumption'. I'm guessing these are foreshadowing, since they don't have a lot to do with the current novel (19 does come up, and leads to a situation which almost seems like a Han Solo moment -- ie, Han being shot at first in the revised Star Wars -- but I think it works better than that).
Also, two points which I think are mistakes (since I'm a pedantic sod): In a later book we are told that 'char' only refers to 'death', and so Roland doesn't know the name Charlie. Yet there is a now a character from Gilead called Charles, son of Charles (but since he is associate with death, at least within the narrative, it might be a strange bit of foreboding). Also, Roland asks Walter about 'glammer', but the dialogue when Walter brings it up has been removed (unless it was moved somewhere earlier and I missed it).
In summary, there are no enormous improvements that have been made, and some things that a fan of the original could quibble at (Stephen King himself says the book's primary audience is not those people, but for newcomers to his world), but at the same time many of the changes have been to the book's benefit. And it's still a powerful novel, a refreshingly different fantasy and a great start to a great series.