Dealer installed semi-metallic brakes and they squeak. They claim that for the 1st 300 miles they need to be broken in for all semi metallic pads. Can anyone confirm this, or is the dealer full of it and taking me for a ride?

 

The squeak happens when air pockets form where the pads seal. If they squeak when you drove out of the shop that's a bad sign, I'd go back right back and let them know, then if it doesn't improve in afew days go back and demand they replace the pads.
If the squeak started later it's probably too late, the only cure is to replace then pads and possibly machine the rotors. The good news is it's just noise, no need to replace them unless you can't stand the noise. The most common cause is hard stops during the break-in period (blame the driver), though unbalanced rotors or cheap pads could also be the culprit.

 

(I'm wondering why they used "semi-metallic" instead of the 'higher tech' ceramic pads we art told the vehicle comes with...)

There is a break-in period for ANY new pads, and it is covered in your owner's manual... gently breaking for the first 200 miles until the pads seat properly about covers it.

Tomas