From what I've read, almost all of those devices (like the Apple newton) were failures, but were there any PDAs made by obscure companies that were actually good and sold well?
This is an odd question and I doubt that you'll find a concrete academic source. Psion-brand devices used to be popular with some UK civil service employees as a personal organiser. While there was no official purchase of those devices, anyone with an accessories allowance would likely end up gravitating towards a pda. I know that quite a few of my dad's CS colleagues had one and the series 3 and 5 was popular. Of course the smartphone killed the pda.
Litchfield, Steve (2005) [1998]. "The History of Psion". Palmtop Magazine.
Well in the short term, palm did really well in the 90s and was the PDA market. I know the medical community loved their palm devices. So in the Short, yes.
However their real legacy is in testing new technologies which eventually found their way into smart phones.
Later the Palm devices evolved into the Palm Pre model, cell phones. After Palm went under they sold their software notable the WebOS which ran the palm pre, to HP. HP now uses the WebOS in all their smart TVs and other smart devices.
In the grander scale the technology that those devices pioneered are still used directly either through software that is still being used or in the indirect sense they all helped us get to Smart Phones.