In the vein of different kinds of players, I found my impressions of the Sandrah Saga swing from disappointment to delight. All it took was for me to adopt a different mindset. Anyone of that certain type of player that Roxanne described, but who's willing to try something a little different, should read on.
I have to admit, I was disheartened -- before I started playing -- to see Sebastian's comments. My own expectations coming in seem like they'd be inline with his, and his critiques were ones that I'd be likely to make. Indeed, when I first started playing (I created a test character just to try to see if Sebastian's critiques had merit), I was very concerned. Events proceeded much as he described and Sandrah seemed to be destined for Mary Sue-dom.
However, I had already invest enough anticipation into this mod that I decided to keep going (with a more permanent Charname, not a generic fighter named "Test"), to give Sandrah the benefit of the doubt.
I'm very glad I did.
There is no doubt that the Sandrah Saga creates a different Baldur's Gate experience than vanilla. In particular, the assumption from vanilla is that the PC is the superstar. Period, the end. The PC has a divine spark, the PC is called "godchild" in the abyss -- no one else. All party NPCs are beneath the PC, in various ways. This is the story of your PC becoming a god. Two of the best NPC mods I've played so far are Finch for BG1, and Isra for BG1&2. They both maintain that balance and, while the NPCs are interesting individuals, they do not steal the show from the PC.
With Sandrah, I had to adjust that paradigm. If I viewed Sandrah as sort of a co-PC -- another superstar -- then her story seems to fit very well. My impression so far is that this story is equally about my PC and Sandrah. We are co-superstars. In this respect I have to say that so far, Sandrah is very well balanced, story wise, and a delight to play.
Mary Sue stuff is rife with controversy, so here's my opinion on the matter -- an obvious YMMV applies. A true Mary Sue would make the story all about her. Sandrah does not. This story is about you both. This is very different from base BG, but nevertheless it is not Mary Sue. Sandrah is a very special character, but so is the PC. She doesn't overshadow the PC, but she does cast an equally long shadow. When I viewed my game and the story in that light, I was able to enjoy it immensely. Exactly why her shadow is so long, I do not know, won't speculate here, and will play to find out.
I should say that I am very early on in my playthrough. I have rescued Dynaheir, but I haven't even cleared the Nashkel mines. If the storytelling trends continue as they are, then I anticipate this will be very different Baldur's Gate, but one I will love playing.
A note on power level: Sandrah, as well as some of the recommended mods (Dark Horizons, I'm looking at you), adds in some very powerful content, by vanilla standards. My characters are mostly early SoA equipped, while still in Chapter 2 -- purely from drops and treasure. It sounds horribly unbalanced, until I look at my character sheet and see that I'm level 6 already. That's one level shy of the start of SoA, and 2 levels shy of the expected level where this equipment would be more commonly available in BG2. Yes, much of BG1 will present less of a challenge. However, there's lots more content that will. I'm intrigued by this new balance. I don't call it unbalanced. Instead I'd call it differently balanced.
If anything, I'd say that it takes that power-gamey feel of late SoA and ToB, and imports it into BG1. There's something to be said for starting slow, and earning your untouchable-ness (it's what I love about the monk class -- weak at the start, godlike at the end). It's the way I normally prefer to play. However, I'm willing to give this a try. It's been pretty fun so far.
Kudos on a very elaborate mod, Roxanne. If trends continue, I'll enjoy it all the way up through ToB (and beyond!), and again once IWD-in-EET comes out 
For everyone else, give Sandrah a shot, but make sure you understand what you're getting in to. It's a wild ride that's well worth taking.
I feel like I understand her character more. Elements that originally seemed out of place make more sense now. I hope Sandrah addresses it with the PC at some point, too, as it was jarring the first time Sandrah engaged in "extracurricular activities".
