I have a MY06 with Eibach. I am not a real suspension whiz like some of the guys here (Red MX5...), but I can say that the handling of the car did change a bit (towards understeer I believe) but the car is still as capable, predictable and fun as it was as stock ride height. The change was not dramatic enough to make me run back to stock. It just looks better now. If you want something that gives a slight drop without changing too much- this is the way to go.
I will say that an Eibach drop on stock wheels is not going to be terribly noticeable however.