Those are interesting thoughts on Suikoden, pierrot. It's not the first time I've seen someone claim the 3rd game is their favorite on the PS2, but it is the first time I've seen someone have such negative thoughts about the 5th game. I thought III had a really interesting story and narrative, but I did not like the gameplay very much and that deterred me from ever finishing it.
I know exactly how you feel, and I'm going to take a shot in the dark here and guess that you gave up around the point you fought one of the red golems on the mountain path in Gedoe's story, perhaps? Initially I hated the battle mechanics, but in hindsight I can admit that it was mostly out of ignorance. The thing that was really souring for me was how easily I was able to collect all 108 SoDs in one playthrough, without any help. One of the main things that kept me coming back to I and II was figuring out where all the characters were, and how to recruit them (and by a certain point in the game) in order to get better endings. After replaying them to death almost, I now have much different reasons to replay them, so the characters feeling 'fed' to me in III isn't the same issue that it was eleven years ago.
I do think the story, which is very good, would be the main draw for anyone starting out with the game, at least until the very end where it admittedly falls apart a bit. The thing I like so much about III is how much character is actually given to each SoD not just in story events and interactions, but in character abilities; and I think that's where the game really shines, for me. Many more characters can be used in battle in a way that is almost wholly unique to that character, and the actual battle mechanics make figuring out those roles more important, as well as party management more important, by adding area of effect, and character movement, along with character actions being tied to a partner. The game is not very gentle to players on their first time, though. I guess one other knock I would give against the game might be that it's also two to three times as long as I or II was, and that is potentially significant. At least, I find it harder to replay III, even though I love it so much. It's not without its faults, but I still find it endearing for them.
As far as V goes, it may not be a popular opinion, but I just find it to be a wolf in sheep's clothing. I'm not nearly as entertained by sequels where the original intent is lost with the departure of the series' original writer/director, though. The developers for V did everything they could to make it feel like it belongs in the numbered series, while making it as marketable and popular as possible. It just comes up short for me, and I actually hate them a bit for even trying. Murayama o nada.
I had a complete blast with V. So you just gotta play it for yourself. Had one of the coolest castles too.
I don't even remember the castle in V. There was something like four or five bases in that game.