Why go for a Porsche when an Mustang will get you 90% there? I like that final 10% (disclaimer: I don't have a Porsche, I have an Audi S4 :14

. My Oly 5060 was a great camera, especially combined with my Ike housing and DS-125 strobe. With Inon wet mount macro lenses, it was even better. Still, my Nikon 60mm lens and Woody diopter is even better than those. And the camera is fast as heck. Is it all 4x better (the approximate cost differential between the two systems)? No. That final 10% is exponentially more expensive. That's all about macro.
When it comes to wide-angle, I expect that my Nikon might be 4x better (at least 2 or 3x better) because of its larger sensor, larger aperatures, and better lens selection. This is my weak area, though and I'm still learning. My wide-angle photography problems are the fault of the photographer, not the equipment.
Another reason that I take my D80 underwater is that I use it for topside photography.
I want Seatool/Sea&Sea/other aluminum housings in the same way I admire the engineering of the Porsche. Its good to look at, but not really necessary. The only big draw for me is the viewfinder attachments that are much better than the Ike viewfinder. Those attachments cost almost as much as a whole Ike housing. I'm not going to pay 3-4x times the Ike housings (which is already 2-3x more expensive than the pns housing) just for the viewfinder.
Like somebody else said (maybe f3Nikon), I don't want to buy a housing that is so expensive that I am still paying on it when it becomes obsolete. With the Ike setup, the most I'll be out is the $1200 housing if I upgrade cameras and I can probably sell the housing for $600, even then. I won't feel too bad if I decide to upgrade. That being said, I think that I'm still good for at least several more years with my D80, although I would love a D80x with live view, a dust shaker, etc., as long as the buttons were all in the same place.
David