The (modern) Springer front end is one of the most comfortable and best handling fronts made for a Harley cruiser, or street bike. People that have never ridden one will tell you it's old and clunky and so forth, but ask any Springer owner and they'll tell you what it's like from first hand experience.
As for putting one on an XL? It will likely handle OK, but probably not as buttery smooth as it was designed for. And there's a lot of other factors too. If you use a DML or a Paughco front end, or if you rake it out a lot, that's all going to effect the handling as well.
If you like that certain look, then go for it! You'll be happy enough with the ride and love the way your bike looks. But if you're a performance rider than you'll likely be a little disappointed.
A Springer front end can reduce the trail if it moves the axle forward of the steering line, which has the effect of reducing high speed stability. If the spring rate and damping is set to a good value for the rake and weight of the bike, and you don't ride 100mph, you may not notice.
As Eye said, the HD Springer front end works remarkably well on the Big twin bikes so why not on a Sportster.
My own experience on the Springer's I've rode is that they works as well or better than anything Harley (Or the rest) makes. The ride is very good and there is no front end dive on braking---but herein lies the only problem I've seen besides the difficulty in cleaning all of that beautiful Chrome (Some are black) and that is the serious lack of front braking power! The single disk is weak and there is no provision for mounting a dual disk system.
Of course, most Sportsters are single front disk bikes anyway. My past one was the 883R with dual front disks as is the 1200R and the XR1200.
If you do it, post some pictures.
You'll get some comments from people that have never even sat on nor rode a Harley, Sportster or not, nor rode a bike with a Springer front end but this is the fun part of exchanging ideas.
Hey Critter, time for you to jump in here beings you just purchased one and would have first hand new owner views. I've never owned one but have ridden them and like the way they ride. I'd love to see one on a Ultra just to help stop the front end dive but I'm thinking it would have to be so beefy that it would look ugly. Plus you got just that much more chrome in tight places to deal with that it wouldn't be worth it. JMHO.
I hadn't considered braking differences due to weight shift. My guess is that the spring resistance could be changed to allow weight to shift forward. Ones that are purpose built for Sporties should already have that considered in the design. Pulling one off a heavier motorcycle with no changes to the springs could cause problems.