Using a CPAP is a night and day difference--whereas I used to require 10-12 hours to function without randomly falling asleep if I sat still for too long, and now I can get 6-8 without any issues and I'm more alert, have a better mood, etc.
I wouldn't have known except that my ex-girlfriend noticed that I wasn't breathing when I was asleep and got me to see a doctor about it. Apparently it had been going on for years and years--I asked other people who had seen/heard me sleep from 15 onwards and a lot of them also had noticed it but hadn't said anything.
If you think you have apnea, sleep studies suck to go through ("here, lets stick a bunch of monitors on you and then let you try to sleep"), but it was entirely worth it. If it's at all economical to do one, I'd suggest looking into it: even if they don't find apnea they might find something else.