I absolutely love lightning mode in Bejeweled 3. Steve's right; it's like Bejeweled Blitz but not as sucky. Getting a good score isn't as luck-based as it seems to be in Blitz, as 3's time gems appear more frequently than Blitz's oh-so-elusive multipliers, and since leftover time gems become flame/lightning gems if time runs out, it helps keep the board flowing for the next multiplier round.
Also, supernova gems are officially the coolest thing ever!
EDIT: I'm blinking at my screen and seeing Bejeweled gems. It's taking over!!!