It's like what everyone has said so far, everyone has their own preference. To some, this last part of the series is one that is great. And the pairing has great chemistry and definitely not stiff at all. To others, this last part doesn't please them, and the acting isn't to their liking.
To me, there are some parts that I feel do lag and aren't needed but in general, I think the two leads are great at their roles. I wouldn't be able to picture anyone else in those roles. Not even Mark...and personally I'm a great MM Team fan. James Ma is doing so perfectly well as Khun Chai Ronnapee even though he does have some bad publicity, his acting shouldn't be one to be too harsh on because he's stood his ground as a newbie.
As for Chalida, she's proved her growth in each lakorn she gets to do, and she's one of those few actresses who has near perfect and great chemistry with all her leading men.
I keep forgetting that the two in real life are only 19/20 years old.
I think those who has become attached to Kwan and Ronnapee are easily satisfied at the cute little scenes and heartbreaking scenes because we've grown to feel their pain almost. And although, it seemed lagging. Unlike most of the other brother's parts of the series, this last part actually goes into depth about the Kwan's family's history with love and higher ranked men along with that knowing that Chai Pee carrying a lot of burden with being the only last possible one to carry out the promise, we feel their pain and most definitely the greater sense of a love that cannot be.
