I use Screen4. It's simple, and once you learn it's intricacies, it works very well. It's not perfect, especially the way the position dialog boxes work.
Machscreen is better, but it's always been unusable for me. I've tried multiple versions of it, and have always had it crash each time I've used it. I've never spent anytime trying to determine what caused the crashing. Since Screen4 has never crashed, I went back to it and stuck with it.
Yes, a new designer would be great, but I think it's usefulness may be soon coming to an end, as Mach4 will have no use for it.