I remember never really using it on my 1st BG2 playthrough. I probably didn't want to lose all of the reputation points and/or I'm guessing my sorcerer just was too powerful late game for this form to really be worth using over his spells (?).
This time around, my main character is a Monk so I thought that since he's already a melee character, the Slayer form might be a much stronger version of my character. I also am not soloing so I don't have crazy high level (but did BG1/SoD with him first so he started at higher lvl in SoA).
Looking at the Slayer form decription it says:
1- str/dex at 25
2- AC -9 (I think it's base AC)
3- 40% magic resistance
4- Saving throws set to 2
5- 4 attacks per round 1d8 + 2 cold dmg
6- THAC0 to 0
7- +100 hp
8- Immunity to imprisonment, level drain, stun, missiles
Now I know that my Dark Moon Monk becomes quite strong at lvl 15+ (currently lvl 19, close to 20). I also used a mod to be able to rebuy some of my SoD items and reclaim my cloak of balduran by killing the shadow dragon (unofficial item pack) so this does skew it a bit because the cloak adds an other 25% magic resistance and the monk helm from SoD allows me to use DUHM which coupled with the stats books from my BG1 run can push my stats to 25.
Still even if I didn't have these, the Slayer form would still be a downgrade on almost everything.
here are the stats comparison between both (using DHUM to buff myself with the SoD helm):
-Dark Moon Monk (lvl 19, close to 20 at which he'll even become immune to non-magical weapons, have higher magic resistances, etc.):
AC: -11 (-20 vs missiles)
Dmg: 21-40 (4 attacks per round, 15% chance for 1D10 cold dmg from gauntlets I got from Rasaad quest). Can be buffed a bit more with other monk abilities like +2 frost dmg for 1 turn and so on.
Magic resistance: 97% (100% at lvl 20, would still be at 72% without cloak of balduran that I got from the item mod)
Immune to poison
Saving throws: -5
*Lose 2 permanent rep per transformation (my character is evil but rep still can help for prices, quests and to keep Keldorn around in my party)
AC: -16 (-19 vs missiles) so 5 better armor in general, 1 less armor vs missiles (slightly better in general)
Health: 100 (so lose 10hp, not sure if description for +100 is wrong or if form is bugged)
THAC0: -7 (3 worse THAC0)
Dmg: 15-22 (5 attacks per round, +2 frost dmg per hit) (looks like the damage significantly weaker)
Magic resistance: 97% (not sure if bugged, if the resistance was supposed to be set at 40 or if 40 was supposed to be added but something's wrong here, still no buff in the end)
Saving throws: -4, -8, -7, -10. -7 (except for one, the saving throws are a bit better especially vs breath)
The biggest buff is probably the saving throws but with such a high magic resistance and poison immunity, most saving throws aren't even needed anyways (and it could be all matched using a single potion). So I just got the form and despite the significant penalty for using it, it seems to already be outclassed by my monk at lvl 19 and since I don't think that it scales, it will only keep getting worse. The only advantage I can think of is the immunity to imprisonment (magic resistance doesn't work vs it I think) and some people suggesting to use it to kill Kangaxx but I've killed that lich at low level using a scroll of protection vs undead for his 2nd form already.
I'm guessing that the form might be more useful for an evil party where the main character is neither a powerful spell caster nor a strong fighter? More useful for something like a Druid perhaps? Do you guys ever really use it apart for giggles and because it looks cool?