The P25 mode introduces frame smears and/or jerkiness when there is a lot of action, eg in fast pans. I do not like it, feeling that it is rather pretentious, emulating the deficiencies of film shooting. My own human vision does not smear, so why should I chase smearing other than going for a retro look. Like Sepia, also included. Each to his own tastes. In low lighting the shutter speed becomes longer and the smearing gets worse with P25. Nevertheless you do gain something like a full f-stop in exposure (maybe more?) and probably worthwhile when you are shooting under very dim conditions. I do not think I will ever use P25 except for the low light situation to get the extra f-stop (in longer shutter speed). Interestingly, in bright scenes without too much action, you can view P25 and normal intercuts without noticing that you have switched the scan mode.
I downloaded a couple of HV20 24p demo clips and noticed the obvious judder. I thought it might be due to some conflict with the 70Hz refresh rate of my monitor. Also 24 fps does not fit neatly into the NTSC 60Hz system, whereas 25p does fit into our PAL 50Hz system with a simple frame doubling, but I can't see any reference to such a feature in any of the video editing programs I've come across, ie. converting 25p into 50p with frame doubling.
The judder I saw in the sample clips is not something I've noticed when watching a movie in the theatre, yet I believe all movies are shot at 24 fps.
This is a new game for me. I'll probably buy an HV20 at some stage but I get the impression there might be some difficulty in getting the most out of 25p due to a lack of appropriate software.