My 11 month old son is currently in a band - he started off with moderate plagio and is now down to 3mm (we are due to finish in a couple of weeks, yay!).
The pros:
They do seem to work quickly
They do not hurt the child and aren't invasive (they channel new growth, rather than changing existing shape)
The cons:
The first week or so in them can be a very tough learning curve - you start gradually building up the time in the hat and whilst it doesn't hurt, the baby has to get used to the feeling. Our DSs sleep was affected for a while
Your baby can get red marks / sores when the hat starts to rub (the consultants shave off the lining as the head grows)
It is a faff - it needs cleaning twice a day
You have to go for regular reviews - and the time and cost associated with this
People stare - blatantly and that is tough (but in the grand scheme of things, unimportant)
I am so glad we have gone for treatment (although I am counting down the days until it is over). His head looks totally different and I am so glad we have done something in time. No mum wants to band their child, but longer term, we felt it justified it.
Good luck with whatever your choice.
I will add that the clinics we went to didn't push us into going for helmet treatment. They measured DS and explained how the helmet works, told us to go and try repositioning for a while and go back if it was still bothering us.
The earlier you do it, the quicker and more dramatic results you are like to get.
Good luck xx