I had hoped to take advantage of some drush-5.0 features, specifically better handling of file permissions and built-in drush make, but I've found that drush-5.0-dev is still a moving target (no stable release) and new issues have cropped up, e.g.
Make cache directory selection more robust in situations where selected location is not writable

For now, I've decided to stick with the latest stable release of drush, drush-4.5, and separately install drush_make-6.x-2.3, although it will have to be removed later when upgrading to drush-5.0.