There's a few topics on here already, but here's the quick and dirty:
You will be stuck waiting for your security clearance doing joe-jobs for anywhere from 1-2 years after you finish your BMQ. During this time, you'll do BMQ-Land and also your downclass (first part of QL3).
For promotions, its on par with other trades.
Spec Pay: Comm Rsch pers make Spec 1 on the pay scale, only after they have completed their QL5s. To get to the QL5 level, you'll spend about 4-5 years in the CF. The working rank depends on where you are posted. Cpl is the working rank at the EW Regt, as they do the lionshare of shift work in the back of the vehicle.