Well I have experimented a bit with this spell, and I was more than impressed. This magical sword has so many great resistances that they can make it perhaps the best summoning! I mean; it is immune to anything but pure magical energy like magic missile. It is a perfect choice against fighters, because they can't harm it at all! AND! It is the ultimate weapon against mind flayers!
I cheated and created five Ulitharids and let the sword attack them. They have massively bombarded the sword with psionic blast and psionic domination, but...no effect..heh..heh :D. This marvelous summ. must be immune to any charm spells. AND! It can be cast COMPLETELY UNSEEN(so the spell doesn't reveal the mage to the enemies) It also deals a good damage, though it attacks only once per round(that's what haste is for ;)). AND its THAC0 is getting better as the mage levels up! Its only weakness that it is wulnerable to the death spell.

So...I can say that Mordenkainen's sword is the best summon(not gate) spell! Any comments?

When Mordenkainen's swords die, in some games they leave a "necklace" - in those (unpatched) games they are NOT immune to charme (bad programming, the sowrd can't activate the anti-charm function of the nacklace); but the sword should be uncharmable, you are right! UPDATE HERE (http://members.home.net/mrkevvy/SoAfix/FixedBG2Mordenkainen'sSwordAmulet.zip)

Being able to summon it while invisible was not intended!!! IT WONLY WORKS FOR MORDENKAINEN'S!!! Exploiting this is cheating! UPDATE HERE (http://members.home.net/mrkevvy/SoAfix/FixedBG2Mordenkainen'sSwordInvisibilityExploit.zip )

Mordenkainens is good but has a few downsides... it is vulnerable to confusion and since the favourite pets of Mind Flayers are Umber Hulks it's likely to get confused and just stand around doing nothing, also it doesn't last very long (1 round/level).

I prefer the Skeleton Warrior... they have high magic resistance and are pretty tough melee fighters, although not as good as the sword, and stick around for a long time.

I think skeleton warriors are better as well, when reaching level 15 they become really nasty. Mordenkainens swords just can't hit anything later on (except for those flayers).

Though mixing them, as having the swords being attacked and the skeletons deal damage is great.

I think skeleton warriors have much more better use in SoA plus coming early also. :)

question: do the skeletons summoned increase in level as your cleric/mage does?

Answer: unlike npcs' leveling up, there are just two types of skeletons summoned: immature and mature. :)

I also like skeleton warriors very mutch, but against the Mord.'s sword is better against the flayers having no intelligence at all. Skeleton warriors do have a 15-16 intelligence. It is written in the Monsters manual. Also, I tested the THAC0 of the skel.warriors and the Mord's swords(on Minsc heh..he.. :D), and I found that Mord's sword's THAC0 is better than the skeleton warriors.