Sonic X, in my opinion, is the best Sonic sereies, despite Chris Thorndyke. The first season gives a possible explanation as to why there are a few accepted anthropomorphic animals living alongside humans, something the games never did do.
However, the Sonic Adventure 2 arc provides one more question: if Eggman is originally from the human world, how did he get to Sonic's world?
Anyway on to answering the thread questions:
1) I think that their romace was beautiful and, if you liked it, you should watch the Japanese version. (The 4Kids dub toned down their romance in the final Metarex battle.)
2) CHARACTER PORTRAYAL
SONIC - A bit more or an attitude towards characters than in the games.(Eggman not included)
CREAM - In the games, she started out willing to fight Eggman since her introduction.
In Sonic X, she was a pacifist until Emerl went berserk.
BIG - Does anyone else think Sonic X made Big the Cat look even dumber than normal?
AMY - Quicker than ever to anger which provided plenty of comedy.
Everyone else seemed fairly accurately portrayed.
STORY
SONIC ADVENTURE
-The battle aginst Chaos 2 occured about 3/4 of a day earlier than it should have, plus it was Knuckles alone who battled it.
-In the game the battle againts Chaos 4 occurs after Knuckles is tricked into fighting Sonic, then, Sonic, Tails, and Knuckles all fight Chaos 4 together, yet, in Sonic X, Sonic alone fights it and Tails is made into a klutz who can't resist a gust of wind.
-Tails' battle against the Egg Walker is completely removed.
3) I think the writers wanted to put more than one romance in the script. Maybe the romance between Tails and Cosmo was well recieved in the writers room and they decided to create another one.