Admittedly, the experiment didn't go well. I make a bitchin' shadow priest, but even with stellar gear, I make a terrible disc priest. That's part of why I don't respec very often.
How about you? Do you change your character's spec to meet the needs of raid bosses several times a night? Do you respec to run heroics or dailies? Or are you absolutely adamant about playing your favorite spec, no matter what?