Hard to place all pros and cons squarely on SCDSL. You have DA/ECNL/DPL/ENCL2 and Birth Year change that have also had a major impact on the game.
SCDSL has allowed for the mega-clubs to become even more mega with mergers/takeovers/affiliations. Still not sure if this is a pro or a con.
Now you have clubs with 4-6 (maybe more if you include affiliates) in an age group.
For teams in OC, SCDSL has minimized travel.
The "self selection" of flights have pretty much gone away. Flight 2 and 3 - no scrutiny. Flight 1 - you need to prove that you belong there. And then you need to prove if you belong in Champions or Europa. I think it's too many divisions in Flight 1. But overall, it seems that the competition is about right. There are some outliers that kill everyone. And some outliers that get killed more often.
Has the soccer improved because of SCDSL? Hard to say. DA and ECNL probably have more to do with the quality. But I'm sure there is some trickle down effect. The smaller community based club is a dying breed. I think that is due to SCDSL. Coast has some small clubs with some really competitive teams. In SCDSL - small clubs don't fare as well. (Look at all divisions of State Cup/National Cup for examples.)
Are more kids playing soccer? I'd say more kids are playing CLUB soccer. But overall, I think less kids are playing soccer around town. Kids that would probably be best served playing AYSO for $150 a yer are now playing on a flight 3 team for $2,500 a year. But I dont think is because of SCDSL. Seems to be the norm across the country. And not just soccer (basketball, baseball, etc).