I've been using a 1620 for about 8-9 months with Rockbox running. I'm having no problems listening to mp3, ogg and wma files, all buttons do what they're supposed to do.
The only issues I have is a poor battery life and if I shutdown with the phones (PC speakers) plugged in the 1620 restarts itself. I cant say what the plugins are like as I don't use them on this player.
To install and to put anything onto the player it has to be connected to the PC using "service mode" (method 1) http://www.rockbox.org/wiki/GoGearHDD6330
Compile Rockbox for 1620/30/35 http://www.rockbox.org/wiki/SimpleGuideToCompiling
Once you get the .rockbox
file compiled open it and take the FWImage.ebn
file and place it in the System folder
(not the "_System" folder) of your player. ** The System folder is hidden so make sure your operating system is set to show it **.
The FWImage.ebn file will overwrite the original firmware file meaning dual boot is not possible but Rockbox is far superior than the Philips firmware plus there is only 6GB of space.
If anyone can improve on what I said please do.
This needs to be put on the Wiki but what I've said is very brief.