For me, the best thing about SR is, that they accepted each other for who they are. Sid accepted Riddhima at her worst, when all she needed was support and Sid gave her that support and love unconditionally. He's the one who taught her to live again, after she was completely shattered, numb and completely clueless of her surroundings. He's mellowed her down to a great extent......
On the other hand, Riddhima's presence has changed Sid completely. He was the brat, and now he's a much more mature and responsible person. Earlier, he was concerned about no one but himself, and now, he's concerned only about Riddhima's happiness....
Their relationship was never perfect, like in movies, but in reality every relationship has flaws, and how they get pass those flaws and problems is the real test of their relationship.