After reading up to Chapter 38(the most recent chapter released when this was written) I can safely say that KissXSis is my favorite ecchi manga of all time. Why is it so good? For starters, the main character, Keita, isn't searching for an unobtainable girl that eventually falls for him. His twin, non blood-related step sisters, Ako and Riko, are the ones in love with him, and they openly show it(even infront of the parents, which fully approve and support it).
The two sisters are what make the manga so popular. They each want to get Keita, they each kiss him ALOT and because of that there's plenty of conflict between the two. The funny part is how each sister tries to isolate Keita and herself, and what they have planned.
There are alot of chapters that push the ecchi to the max, where one more step and it would be considered Hentai. Look at it this way: By chapter 10, Keita has gotten dragged into situations by either Riko or Ako that exceed everything that other ecchi manga like to-LOVE-ru and Love Hina did in all their chapters.
Another thing that should be noted is that this manga loves to poke fun at relationship situations looked down upon by society, such as Student-Teacher relationships.
At this point
By chapter 8 or so(don't remember which exactly) Keita has a flashback of when he was young and how he wanted to marry both his sisters. His father replies by saying that it's illegal, which saddens the young Keita. In later chapters, he openly admits that he loves them both, which makes it clear that Keita will have to make a decision one day: Who will he choose? The sweet, closet-pervert Ako? Or the tough, honest Riko?
It's because of the decision, and how they both are given the same amount of focus, that I really hope it doesn't have a Harem/improper ending. to-LOVE-ru, a series I fell in love with, had the same problem with the ending. Keita needs to choose one of his two sisters, and we, the readers, should get a foreshadowing when the manga nears its end, about who Keita will choose. His sisters love him too much, and he loves them too much, for Keita to not pick any.
In short, excellent read, every chapter forces you to read the next!