Honestly, Madworld isn't my game. Too bloody. But it's absolutely stylish and that's what sells on Wii. HotD may be an on rails shooter, but it always was one and HotD is allowed to do one, since that's what it's all about since the beginning.
It isn't zombie-slashing, that sells HotD Overkill, it's the hilarious connection and relationship between Agent G and Washington. The Cutscenes and the over the top acting is what makes this game believable even when it's zombie slashing is way over the top. Wii owner love that. I love that. I laughed my ass of about effing of Washington, about him calling G a sissy, about his accent, about... everything. They also give a lot of tips making the boss fights easier then House Of The Dead arcade machine did. HotD 2 arcade was short, hard and sterile. Overkill is fun, long, crazy and great.
I think Madworld is similar, maybe if only I would play it it would give me pleasure. But the style is unique and that you can see without plaing it. I love it's style!