Wouldn't a Marilith with 6 Speed Longswords be awesome!?

[Keld Denar]

I agree with Meepo. Here it specifically says "extra attack with it". Each weapon is clearly distinct.

"the wielder of a speed weapon may make one extra attack..."

If you look at it like this, the property is affecting the wielder. If its affecting the wielder, then each additional weapon of speed would also affect the wielder, and thus would not be allowed by the "similar effects" clause. The "with it(the weapon)." could be seen as a restrictive clause that says that the one extra haste attack you get HAS to be with that weapon, unlike a normal haste attack which can be made with any weapon you are wielding.

Also, the easy way to have the Marilith make extra attacks is to change two of it's feats to Improved Multiweapon Fighting and Greater Multiweapon Fighting. Or heck, even Improved Unarmed Strike would give it 6 extra attacks per round.