I'd love to reply! Turbo Mode is still new to me, so please don't take this as gospel. Feel free to correct me, and such.
Coming from a primarily Brawl-based Pikachu background, I use my dsmash all the time. It's a momentum stopper, especially against close-ranged characters like Sonic, which need to be in a character's face to wreck havoc, and when they do, they are terrifying to deal with.
All of my combos tend to start with a D-smash (though downthrow works more predictably, you need to be closer in), followed by a jump towards them, into F-air's, and end with a D-air, or an U-air. I use the D-air when I'm close to the ground, for a quick hit-stun, so that I can continue ground combo to death, almost always ending with a thunder.
A good video to watch is of the Project M Turbo Tuesday Finale. 1:25 seconds in is the Pikachu turbo.
In order, it's Quick Attack to 2 jabs, to running jab, downthrow, 2 U-airs to Neutral B, U-smash to Thunderkill. Looks really situational, but the Quick attack entrance looks great, especially without the lag into attack, and the downthrow to up-air and thunder kill seems really consistent as well. However, with the speed of Turbo Mode, I think in the time it takes to think about it, you've already lost the chain. I guess just let the fingers go!
