I think that Endurium and Ryan333's posts basically sum it up well. If you're a completely new player who's never experienced the BG saga before, the EE is not a bad starting place. It'll get you into the story and game with minimal fuss and without needing to worry about mods or compatibility issues, and if you've never played the original game, you likely won't even notice the differences between the old and new content. In addition, the EE's use of the BG2 engine adds a bunch of fantastic QoL improvements such as the ability to highlight interactable objects, and the option to pick up loot from slain foes without needing to actually walk over to the individual corpse in question. (You can disable this option if you're a real looting purist, don't worry. ;) )

As for Siege of Dragonspear... My opinion is that it's not THAT bad, but it definitely doesn't match up to the standards of the original games. My biggest complaint is that the maps feel too "packed", like there are way too many encounters jammed into individual maps. Like, I can defeat a bunch of ogre bandits, walk 30ft and right into a crusader encampment who mysteriously has no issues with that same bunch of ogres setting up camp within a stone's throw away despite the fact that the two factions are opposed to each other? And not to mention that this is all in woods that are infested with Displacer Beasts that would absolutely shred your typical supply caravan escorted by low-level guards, yet the supplies for the crusaders always get through with no issues? I can only put this sort of issue down to "there wasn't time to create enough secondary maps to spread all of this content out in", which I don't really blame Beamdog for, but it does feel like they missed out on being able to deliver a more polished experience. The overall story of Siege of Dragonspear is entertaining enough, however, and I think it's still worth a look if you pick it up when it goes on sale.

EDIT: One thing though. DEFINITELY install that mod that restores the the original game's cutscenes. The new EE cutscenes are much less appealing, even after accounting for the dated 90's 3D technology. The cinematic that plays after you defeat the final boss of BG1 MUST be experienced in its original form, or it simply won't have the same impact.

Thanks for making this point. I have to restart my campaign (after losing all my gamesaves on a corrupt harddrive), and I have both versions. I shall ensure I re-start with the original version, or with the EE version complemented by the original cutscenes. :)