In the context that the current 1.2.3 code base is beta+, I always upgrade to a different CF and put the old one aside in case there are problems. I agree that any upgrade that requires a screwdriver is not fun. I figured out today that web-based upgrade works if you manually download the image and "upload" it to the router through the web interface. Auto-update is still broken it would appear, but the manual workaround will allow me to retire my screwdriver. ;D
If you write a disk image to a slice and then fsck and/or label the slice (what the pfSense upgrade logic does), you could get a different checksum than that of your original image. The logic in the upgrade has you booting off the slice that got the upgrade. The fall back is to boot from the known good slice. How much effort is made to determine the success of the alternate slice upgrade could perhaps be a matter of debate. IMHO, stability of the recent builds has been quite good.
With ALIX, there's just no substitute for a serial connection at boot time.