DDO TWF. How good is it?

Well, it looks pretty damn good. Essentially your trading a bit of armor class to be able to get DOUBLE the attacks.

1) The difference between Improved TWF and Greater TWF (and the further in
the future Superior TWF):When standing, by default, your first attack when
standing still is right hand only, your second is a double attack, your third is
right hand only, your fourth is right hand only, and your fifth (which you can't
get to until the level cap goes up a bit) is a double.Improved TWF makes the
third attack a double; Greater TWF makes the fourth attack a double, Superior
TWF makes the fifth a triple (two attacks with right hand, one with left).

So... With Regular TWF you get 1,2,1,1,2 with max attacks. So if you get 3 attacks a round, Regular gives you 4 attacks.

With Improved you get 1,2,2,1,2 with max attacks. So if you get 3 attacks a round, Improved gives you 5.

With Greater you get 1,2,2,2,2 with max attacks. So if you get 3 attacks a round, you still only get 5 but 9 overall compared to regular fighter at max level who would only be getting 5.

And with superior you get 1,2,2,2,3 with max attacks. So at max level you pull off 10 attacks to every 5 of a regular fighter.