My DM imposed this variant in our current campaign. Personally I don't like it. He instituted the rule after a few combats where my sleep spells combined with the charm persons from the sorceror made for 1 round encounters. Both of us had spell focus resulting in high DCs. He thought it was cheesy that we didn't even have to roll anything to be successful. Now that it is random, I find myself choosing spells that don't have a save, so I can be more consistent.