Flake, it's nice to have image stabilisation with the heavier lenses. Like auto focus, I don't know what I'd do without it these days.
I have heard that the 50 EX DG has issues with front and back focus. My copy doesn't seem to have this, but I printed off a focus chart and will run a couple of tests sometime today, and get it calibrated if need be. You can download the PDF file and print the simple chart from here if you want to test your own lenses for this anomaly:
http://focustestchart.com/chart.html
Just scroll to the bottom of the page to get the file, or give the whole article a read first.