I've only played a Houdini prototype and it was a looong time ago so all I can say is it looks really cool and seems like it has potential. The reversed flippers thing was pretty cool.
TNA is a friggin blast. I have a bunch of solid state games and it feels like an updated throwback. It has a unique lock and a great soundtrack and you literally want to play it over and over. Winning!
I played an Alien at the last York show and I really enjoyed it. Don't know if it was a prototype or production model. I think it has a ton of potential as well. I love the look of it and the shots are definitely challenging. That said I would only buy one if it was sitting in front of me.
Honestly I think they're all too expensive but that is the state of things these days.