In case ya come back...I know it has been a month but....
I don't have the UX50 but just recently got an nx73v and use it in landscape mode which is equivalent (480x320 display) to the UX50's standard layout. It reads fine in that mode. I run Plucker, eReader and Mobipocket for Palm OS without any problems. I also have Adobe Reader for Palm OS installed which works good enough, as I rarely have the need to read PDFs.
I will say that larger images are difficult at best and I would not want to read anything with loads of images. I find I loathe to pan around to see the full image so I usually just skip them all together.
So, if you are looking that might be a decent enough device if the cost is cheap enough. That was the ONLY reason I bought the nx73v...it was super cheap. If you are looking at over $150 (for an almost new or new device) then you might consider a dedicated ebook e-ink device instead. But, I find I enjoy the PDA functions quite a bit...more so then I ever imagined I would.
Also, good luck if you want to try to run Opera Mini 4. Opera Mini requires the IBM Websphere Java runtime and it is hit and miss to find or get that running for earlier releases of Palm OS 5.x (I read that it runs well under v5.2 and above for the most part.)
Sorry if this info is a day late and a dollar short...