I have a 7D and doubt seriously if I will ever switch. Why not?
A combination of personal style and overall investment.
Switching to a 5D (or other full frame) involves not only switching the body, but also switching lenses. I would need to invest at least another $1,000 or so over and above the cost of the 5DII to get a wide angle zoom that covers the same range of lenses I already own for my 7D.
I don't do a lot of low-light photography. I don't usually shoot high ISO, and the image quality of the 7D is just fine at normal ISOs. Besides, I am certain that the next generation of 7D will have improved ISO performance anyway.
I print most of my images full-frame, and absent radical crops, the image quality of the 7D in print is flawless, even for relatively large print sizes.
I am spoiled by the extra reach of the 7D. I just bought a 100-400 zoom and I don't want to give up the 1.6 crop factor.
I've studied my own preferred style and composition. I'm not a romanticist, my style is more documentary and my compositions tend to emphasize a flat plane. Thus, I don't find as much need for shallow depth of field and in fact, prefer the greater apparent depth of field of the APS-C sensor.
I'm not prepared to say I will never switch to full frame and the low price of the 5D II is tempting, but for my personal style of photography, at this point I'm not seeing sufficient advantages to full frame.