Are they worth getting? I'm having a hard time moving away from my Praxic Fire chest. How does the extra scorch work with a overshield? Does it either wait for the first shield to go down, stack the shields, or does a second punch while the shield is up just restart the counter?

It just resets the counter. It will give you a fresh shield. A high strength build with grenades reducing melee means you can be shielded almost continuously. It also has special weapon reloader. I've found it to be extremely tanky, especially in the crucible.

I'll add that, if you like cqc or use shotguns, these are life savers. If you're a troll like me and use Pocket Infinity, these are perfect: you reload faster so there's no reason to not use perfect balance, and if you need to actually use your melee, you'll have a shield afterword, Period. I find I spend even less time dying and more time high-fiving people into immolation.