The X5 finally arrived yesterday and I've spent the better part of the evening A/Bing him and his younger brother.
Synopsis of impressions:
X3: Overall the sig (FW 3.0) is more consumer-friendly: thick bass and decent treble, albeit a little rolled off. Mids, esp female vocals are more forward compared to the X5. Newest FW is a nice improvement over the last official release. Very good SQ that few outside of the audiophile community could find issue with. SS is slightly closer to the listener than the X5.
X5: More details and blacker background with more space. Mids and vocals are more in line with the extremes of the spectrum. Depending on your genre of choice, this may be good or bad. There's not a night and day difference from the X3, obviously they're from the same family, although the X5 is noticeably more capable.