I've had reports that Snow Leopard declares hard drive's SMART status failed and therefor will not install.
Was starting to put snow Leopard on my Macbook pro and it has an error
S.M.A.R.T. the hardware problem can not be repaired.

One client reports Snow Leopard breaking EyeTV by Elgato.
So, I installed Snow Leopard on both hoping that would improve things.  The MacBook Pro is still finishing up but all looks good so far.  The first thing I have noticed that has not come back on board properly is the EyeTV.  It just says it is not responding and the rainbow dial just spins forever.   I have Force quit it and restarted the iMac but still no success.

Furthermore:
Snow Leopard broke my own website design tool RapidWeaver. Had to move to the beta of the next release. I'm experiencing tremendous instability on multiple machines. One odd incident however. HP's new Photosmart Plus printer setup utility would not install the wireless functionality of it's printer today via leopard but was successful via Snow Leopard. Weird.