I've done two basements - one framed in all wood and one framed in steel and wood.
There is nothing wrong with mixing steel and wood studs. The wood studs fit neatly into the c-channel. Fasten with a #6 1-1/4 fine thread drywall screw. I used wood to frame the door rough openings. And then used a regular pre-hung door from Lowes. I used wood where I knew I was going to hang cabinets - then I learned how to reinforce the steel.
Regular shelving is no problem. I'd wager most people never find a stud for their shelves anyway.
The plastic grommets are very cheap. $10 will buy more than you need. I can fill a wall of studs with them faster than you can drill one hole in a wood stud.
Yes you screw the outlets boxes to the studs. Meanwhile, Gary is still drilling holes for his sheathed electrical cable. Gary makes it sound hard and time consuming. Its not. I found it easier than the 2 I nailed to wood studs. I overdid the outlets, too. I have one every 4 feet, not the 6 feet in the code.
Yes, I did use wood blocking in a couple of places - but so what? I used them for my plumbing and a 3-gang switch box. A piece of plywood and a couple of screws and I'm done. Meanwhile, Gary is still drilling the holes to run his sheathed electrical cable. Another benefit - I can put the switch box anywhere I want - I doesn't have to be on a stud.
Steel works just fine around a window. I wasn't building an outside wall. I framed around the existing windows with steel - very very very easy! Gary is still nailing the first and I'm done with both. We used wood for my bro-in-law, but his window is one you can sit in.
I have two closets built with steel. I used wood for the rough opening and regular pre-hung doors.
Perpendicular walls - attach a c-channel to the joists above. Easy. So easy, its almost sinful.
I used aviator tin snips - about 100 studs. A metal cutting blade in a circular saw would have been faster. Still completed the framing days before we could have done it with wood.
Screw heads sticking out? yes, but you can't tell it. The drywall is as flat and smooth as if the screws weren't there. BTW - have you looked at wood studs lately? Try to find a dozen that are not bowed or warped! And Gary is worried about the scew heads? Steel studs are straight and true. You can't get a flatter wall.
Insulation IS just as tight. Gary - you are supposed to staple the kraft flaps to the studs. Most of my insulation stayed in palce with friction. A little adhesive on the kraft flaps helped where it was needed.