Interesting character, nice accent, good personal quest. He has two main drawbacks, however.
First, he is basically incompatible with Anders. If you don't like Anders, fair enough, but if you do, it's fair warning to say that you cannot have both Anders and Sebastian in the last part of the game because they are on different sides and you have to choose. One or the other will be gone from the game depending on your choice, even if they are both at 100% friendly.
Second, Sebastian is an archer rogue (along the same lines as Nathaniel Howe in Awakening). I personally didn't have much use for him because I perferred Varric, and by nature, Seb's not a high cunning rogue and thus not great for traps and boxes.
Consequently, in my first run, he spent most of the game chilling in the Chantry and then left during the final chapter.
Also worth bearing in mind that his romance is not as fully developed as the others so don't go expecting the level of interaction you have with the other characters.