I should get my wm5 upgrade cd today but after reading about slowness on the forum i was starting to wish i wasn't. However i found this link to a review for an ipaq 2490 which benchmarks x50, x51v and ipaq and clearly shows that certain things like accessing program menus take 6x's longer in wm5, but apparently other things like video playback and battery life are better in wm5. Since i fly alot (just flew 11 hours on sunday and used my x50v to watch flightplan and listen to mp3's the whole way (with noise cancelling headphones awesome combo) with a standard battery and a double capacity battery getting me barely through the flight) i will give it a shot.
scroll to the middle of the review:
http://www.mobiletechreview.com/ipaq-hx2490.htm