Having satisfactorily used MS Reader and uBook over the last year on my X50v, here are my first impressions of MobiPocket 5.0:
* It installed itself to main memory without offering the option of using a storage card
* It installed a shortcut in the Start menu without asking me, again, very rude for a new app
* Poor marks for folder navigation, the "Library" screen doesn't seem to be able to see subfolders in main memory or in storage cards.
* The "Browse" function in th pull down menu isn't that intuitive to use, nor is the "Go to page" slide bar, but I guess that just another way of saying they do it differently than MS Reader or uBook
* Getting out of Full Screen mode required reading the manual... another sign of a less than intuitive interface, but it turns out MobiPocket relies on the D button as much if not more than the touch screen.
I'll do the 14 day trial and see if it's worth keeping. I've gotten used to the more customizable, user friendly and intuitive uBook interface.
YMMV