Linchpin may or may not be a betatester really running OS4 on his 68k, but he omitted to mention that the 68k bits that will run on 68k are "not the full quid". To get the full experience, you have to have the new Exec and Intuition, which are available only in PPC.
And anyway, it's all installed on top of OS3.9, so you STILL need the OS3.1 ROMS.
I agree with Nate, it's worth the expense.
As for fewer old games working, I don't know, but since the differences are mainly in datatypes and bug fixes, I don't think it would affect them.
tony