So I've just finished up Chapter four, and while the stupid junkyard area has been my least favorite thus far, the interactions between the two teams of characters is really mind blowing.  

First I gotta say I really like how they split everyone up into these smaller teams, despite any negative effects it has on the combat. Secondly, while the Lightening-Hope combo took a while to really get used to (I still can't really decide if I love or hate Lightening), it's turned into something really neat to watch develop.   Sazh and  Vanille, however, have this way about them that just makes me want to keep on playing with them.   So much of their interaction leaves me smiling and craving for more. I can't wait to see if the banter they have keeps coming when the full party meets up again.
 
I was wondering if I was alone in this, or if anyone else finds their chemistry as irresistible as I do?

Hm, I think its great. When you dwindle down the number of people you control or have in a party, it gives them more spotlight and makes the player get to know the character better. The banter is pretty cool I guess.
 
Though combat wise its not so great when you're one man down. The Junkyard level is indeed pretty awful; especially because of those robots which takes like two minutes to kill. I told myself to fuck it and avoided those robots at all cost.

Yeah I'm really digging these smaller dynamics. It's definitely something we haven't seen much of before from the series. 
There are certainly some pacing issues, but the act of developing these characters early on in the game really makes them a lot easier to relate too. Compare this to most FF games where you only learn about characters when you're two thirds of the way through the game (Zidane, Barret, Everyone), or worse, no explanation/motivation at all (Vaan, Penelo). 
 
I went into XII expecting Hope to be godawful. His character design just seemed unbelievably formulaic (Vaan meets Tidus), but he's extremely well done! Sure, he whines a lot, but frankly he's probably the first whining character in an FF where I actually "got" what he's whining about. He's confused, I mean, he's just a kid!
 
Snow still hasn't grown on me >_>

Without a doubt, Snow is my least favorite character so far. Hopefully he'll get fleshed out soon.  Also, in the scences with him and Serrah...he's so much taller than her. It's off-putting to say the least.  Oh well, can't win 'em all.

Some pairs are better than others. Hope/Snow is just horrible as they bring out the worst in each other. Lightning/Hope and Sazh/Vanille are really good, though; the younger characters draw out different character traits in the elder ones. Mama Bear Lightning is just effing heartwarming.

There are definitely clear water shed moments when character dynamics are significantly changed, and given what the complaints above have been focusing on, I'd say for the better.

At the 6 hour mark I have to say I still don't really like Vanille. Sasz is great though, and I don't hate Hope as much anymore.

I actually really like Snow, he doesn't develop a whole lot til later since you don't get to use him much earlier on, but I just love how he is a big teddy bear.  I like the Snow/Serah interactions a lot, it's very 'awwww'.  As opposed to Lightning, who is just a cold-hearted bitch really. 
 
Generally, I like all the cast, Hope aggravates me some and Lightning just seems overly cruel.  The Lightning/Hope pair is just borderline unbearable for me.  They were the same way in the Japanese, and I was hoping the english VA may soften the tone some.  They still come across as bitter to me.

I think the split generally works for helping develop the characters, but the combat really takes a hit because of it.  I really don't enjoy only having two characters and monsters taking so long to go down.  Worst still, sometimes I was unable to actually win. 
 
The story and the characters are quite interesting I find but Hope to be really annoying.  I don't like him at all and the section with him and Lightning was much longer than it needed to be.