I down loaded and played DSOTSC a while ago. IT is pretty buggy I have to say. The first "mission" that I was given umm?, well basically the bad guy that I was supposed to kill just "disappeared" off the map, preventing me from completing the mission and any further DSOTSC ones. I still met a couple of the new (overpowered) NPC's though. I know that their portraits have been borrowed from IWD, their voices could have been too.
Overall I would recommend that you do download this expansion. I think I just got unlucky with the "disappearing" monster and the besides DSOTSC didnt screw up any aspect of BG with bugs or anything.
One thing to keep in mind though is that some of items in this game are RIDICULOUSLY overpowered. Personally I loved these new items (this was my 3rd time through anyway) because they added more exploration and discovery to the game. But like I said, they are very powerful.
SPOILERS!!!!!!!!!!!!
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Some examples of these cool items include:
*A +4 AC ring for druids
*2 different mage cloaks with good AC bonus, lots of extra spells but only usable by Evil chars. Edwin looks exacly like Darth Maul when he carries his glowing staff (+2?) and one of these full body (with a hood!) cloaks. My mages looked absolutely AWESOME and they were soooooo powerful!
*Two handed axe +4 or 5 (cant remember). This thing also had an AC bonus and is possibly the best melee weapon seen in a D&D game IIRC. It can only be used by evil dwarves though. *cough* Kagain *cough*
*There was also some metallic blue armor that you could get. It weighed virtually nothing and I think it was -2. It really was beautiful
. There was also some other top armor that I cant remember the stats for right now.
Seriously, there are so many cool new items to get in DSOTSC (the above is just a small sample) and it made the game a lot more exciting for me going through and finding all this stuff. If you are worried that these items will make the game too easy for you then I would say that you have good reason to do so. I could easily have finished BG1 with pretty much anyone in my Evil party, solo, because of these new items. If you've already been through once though, you should try DSOTSC jsut to make the replay a different experience.
Use an Evil party too, 'cause like I said Edwin looks so awesome in his black cloak as does Kaigan in his metallic blue armor, armed with a huge 2 handed axe.
Dark Side of the Sword Coast... 
Linear Mode
