I also gave it a try and can basically confirm Analogus' experiences.
The .css fonts-workaround in PRS+ certainly is not as good as ebookapps' font integration, but then, I don't change fonts every five minutes. Also, the epub reader did not show embedded fonts in my little
fonts-collection book.
The file browser is good, but collections are just the far better way to organize your books (show me a music player, that does not support id3 tags).
It certainly is great to be able to install custom dictionaries, at the same time, the preinstalled dicts of the original firmware cover my needs just fine. The free Stardict EN-Ger. dictionary (have it on my PB-302) honestly is not helpful (yet).
I'm sure there are situations, when this firmware is to be preferred (eg. having lots of different formats, without wanting to convert them, reading mangas, really striving for open source / free software).
My own needs are met better with PRS+ so I reverted back.
Nevertheless: Good job so far; good to know, there is an open source alternative.