it's like anything, there are good staff and bad staff... in my experience (and I can into contact with a lot of people at cahms) the good people far outweighed the bad and you can of course always complain and request someone else if they are truly terrible
the only thing that sucks at the moment is the short staffing but once you get an appointment that shouldn't actually affect you too much
other posters have told you a bit about the initial appointment, you'll have the option of being alone or with a parent, they will probably want a parent in for at least part of it for family history/their perspective, but they will also give you a chance to talk alone in case you want/need to disclose abuse/self harm/etc without telling your mum (although normal confidentiality stuff applies, they will pass on information if you are at risk), obviously they won't necessarily expect you to disclose heavy stuff to a total stranger so don't worry if you don't feel able to say everything at first (although it will help them give you the right help)
I would recommend giving 100% to whatever therapy they offer you as it really can make a world of difference, but you will get in what you put in, attending appointments wont fix the problems you're having, you have to do that yourself, I honestly think staff at CAHMS saved my life, I think I would be dead by now if I hadn't received the help I did