Gnome 2.6 final isn't out yet, so I haven't tried Spatial Nautilus out yet, but I have attacked the preferences on my office Mac to turn back on the spatial features of Finder (shouldn't that be iFinder?). I have to confess, it isn't as bad as I expected.
The first thing to note is that I tend towards have large numbers of nested directories, this could very quickly lead to the desktop being cluttered with hundreds of windows. Mac solves this with the Alt key, holding it down while one opens a directory leads to the old window closing a few moments after the new open opens. The visual effect is very cute (as the new window bursts out of the old folder icon). I believe I read that Nautilus is implementing something similar, using the middle mouse button (thank goodness - I hate keyboard + mouse combos), I wonder if there will be equally cute window opening effects?
I was going to comment on what I saw as a failing in Finder - there didn't appear to be any quick way to open the containing directory. The options seemed to be 'going through the menus' or a two key keyboard shortcut. Then I found the option, a 'path' icon that lists all the ancestor directories. I remember reading that Nautilus has something like this in the lower left hand corner, the author of that article complained that it didn't look sufficiently button-like.
This, combined with the alleged speed improvements, might be enough to get me to start running Nautilus full time. Perhaps.