During my most recent playthrough of Baldur's Gate, I decided to use SCS. I've finished Baldur's Gate 1 with it, and I just wanted to give my opinion. I didn't use any of the difficulty based components. I wasn't looking for greater challenge, only smarter
opponents (which of course would add challenge). On the whole, I think I like the mod, but I don't think I like the Smarter Mages. I was playing BGT, not EE.
The issue isn't that every mage was absurdly powerful, or that mages were using their spells too efficiently. The problem I had with it, is that every single mage seemed to have access to the best spells of any given level, which I simply couldn't have access to. Every mage of sufficient level seemed to have Stoneskin; but there were only two scrolls of Stoneskin in the game, which meant my party couldn't even all have Stoneskin, because we had three mages. Every mage seemed to have a Minor Sequencer active, and there was only one scroll of that in the game available to me. And far, far too often did I see mages with Minute Meteors, which weren't in the game at all! The Readme said that a scroll of MMM was supposed to be added to Sorcerous Sundries, but it simply wasn't there.
It was a little absurd. The mage on Balduran's Isle, Daese, had Stoneskins and Minor Sequencers. Where she found even a single scroll, after generations on this island, I have no idea. And Nimbul? Let's just say I shouldn't have gone through the Nashkel Mines on level 1! But I have no problem with Nimbul; he should be rather difficult, and had I encountered him later, at a more reasonable level, it would have been a decent challenge, rather than the one sided slaughter it was (so many, many times). One of the enemies guarding the exit to the first level dungeon in Durlag's Tower was a bit unfair; he had some kind of instant invisibility at will. He wasn't a huge issue, but it was wildly unfair.
Almost every time I faced a mage, it felt a bit more frustrating than fun. The rest of it, I loved. Especially the better calls for help. No longer could I have a fight in one room, while the enemies in the next patiently awaited me. Much, much better that the original behaviour. All in all, it was alright, and I'll likely use it in the future; but I was torn between whether or not I'd install the Smarter Mages component in the future. I got my answer in Baldur's Gate 2:
I really wasn't having too much trouble. All the enemies were slightly more challenging that in the vanilla game, but I hadn't found an opponent who could truly stand before me, until I went to the Windspear Hills. There, I met the Ruhk Transmuter. He was so stupidly OP that I simply couldn't take him. Had he no spellcasting ability at all, he still would have been one tough cookie; he was striking on a roll of 2! I only had -4 AC (which is, of course, a bit rubbish), but still; he was also doing a ton of damage, a minimum of 15 per hit. 3.5 APR. Total beast. Then, of course, we have his spells. It was a mess. There was simply no way for me to beat him.
My best attempt was when I used summons to blatantly cheese him, having him waste most of his spells and both of his sequencers against them. Even then, I still had no chance. He teleported into the midst of my group, and started carving us to pieces. No one died, but Charname was my tankiest character. I used four Stoneskins, along with five Mirror Images trying to take him down, to no avail. I got him to Barely Injured, from the minuscule bits of damage hitting through his Stoneskins. After he brought down all of Charname's defenses, I put Nalia in his way; he carved through her defenses, the several Stoneskins and Mirror Images. Same with Edwin. Same with Aerie. Every time his Stoneskins went down, he had another up in an instant. Being a Rakshasa, my spells were useless against him, as we were on too low a level. Barely Injured, and it took everything I had. After that, he quickly finished us.
I decided that it wasn't for me. So, I uninstalled SCS, loaded the autosave outside the dungeon, and completely destroyed him. It was a joke. He didn't even have a Stoneskin, but even if he did it wouldn't have mattered. He wasn't hitting on a 10, and when he did hit, I only took 7 damage. He died before much else happened.
I recently reinstalled the other parts of SCS, but I don't think I'll ever install Smarter Mages again. It's nice that mages are more powerful; the really ought to be. But I think it takes it a step too far.