I'd be thrilled with that trade. I think Monroe is by far the best Center that is available right now. He's still young, he's close to a 7 footer, and IMHO he's been playing like a borderline All-Star for about 3 seasons now.
I don't think it's a stretch to see him becoming a top 4 or 5 center in the league in the near future.
I do believe he's better than Sully right now, and I believe he has higher upside as well. I think Sully's upside is that of a borderline All-Star (think All Harrington or Carlos Boozer at their peak) whereas I think Monroe will be a multi-time All-Star (think Chris Bosh), especially in the East.
If we could get Monroe I'd do it in a heart beat, and if it didn't work I'd keep him and trade Sully out for a more athletic, defensive minded PF who better compliments Monroe. To be honest I think Josh Smith + Monroe would make a great PF/C combo, but I think the presense of Drummond ruins that, as it forces both of those guys out of position.