I have a 60D and I just got a 5D Mark II yesterday. With only 15 minutes of shots of birds and flowers with the 5D, I can immediately tell a difference in the images. With the 5D, the light rolloff is smoother, the depth of field is more isolating and almost three-dimensional, and the shots are more enticing to look at. This probably sounds excessively flowery, but it's difficult to describe the differences unless you shoot with both full-frame and crop. If the 60D shots look good, the 5D shots look great.
Thanks for your impressions. Actually, your description sounds spot on. I actually went ahead and picked up a 5D Mark II last night as well. Since it was already dark by the time I got back home, I didn't get much of a chance to test it out, but what you say about smoother light roll-off, and what others have said about the brightness of the viewfinder I've found to be true.
Actually, I feel I got a pretty good deal here in Japan. A brand new Mark II is selling for 162,000 yen (at today's exchange rate it works out to about $2000 USD; but I still prefer thinking of it as a 1:100 ratio, ie. $1620). The 5D Mark III just isn't what I want for the price I want. So to answer the question posed in the title of this thread: Nope, not for me.
I don't know if I will go back to APS-C and get the Rebel T4i at this point. But with the rumors of 4 more DSLRs on the way this year, maybe there will be a genuine successor to the Mark II, both in capabilities and price. But for now, I'll try to get the hang of my new camera and find out it's limits for myself. Hopefully, by the time I do, I'll have more options to upgrade to.
Thanks again for all your helpful thoughts and insights!