Definitely worth it.
An interesting note is that if you've played BG2 you'll recognize a bit of the map in Durlag's tower - the area where you rescue Haer D'Alis from is one of the upper levels of the tower.
Just an amusing bit.
The TotSC expansion adds a lot of new spells, and items, and the like. It also raises the XP Cap to 161,000 which allows you to get to higher level abilities, and spells. Further, it means that if you export your char for BG2 they start with more XP.