It's definitely worth playing. It's a tad unbalanced between the different classes due to all the special abilities certain folks get and the level cap being so high that single-class thieves and clerics have "wasted" XP after level 40. The itemization however is MUCH more balanced, at least in the sense that certain armor types and weapon types are no longer the best of the best and others are garbage. The story is pretty good as well, and though it doesn't have any truly memorable villains like Irenicus, you do finally get the chance to get closure for the whole "Bhaalspawn" thing. A lot of the balance issues are simply due to D&D not being balanced at super-high levels, though. It is nice that they finally allowed druids to get past level 14, though. (Again, due to weird D&D rules... there can only be one 14th level druid at one time in a geographic region, and there can only ever be one 15th level druid on the ENTIRE PLANET.)
Watcher's Keep is one of the best and most memorable dungeons of any RPG I've played, computerized or PnP, and it can be accessed any time from chapter 2 onwards. The first couple of levels are relatively tame, but the deeper down you go the harder it gets, so you likely won't clear it completely until after SoA is complete, or at LEAST until after chapter 6.
There are a few engine improvements as well, and with Wild Mages and Watcher's Keep and the potion and ammo bags, it's worth having simply for the additions to the main game. I have never regretted getting it, though as a chronic restarter I generally don't play all the way through it before heading back to Irenicus's dungeon with someone new.