I've been to one writing retreat - which I will refrain from naming.
As is standard with any retreat, event, whatever, they advertise themselves as a great escape, a way to spend time eith like-minded people, to learn, etc. They even go so far as to say "if you're an introvert and don't feel comfortable around crowds, we understand - we're like that too." They claimed that all past attendees raved about how wonderful it was, how they had made life-long friends (despite it only being the fourth installment).
The truth is… how well you will get on at a retreat, how like-minded you will find others, depends on you. It isn't a function of the retreat. The attracted crowd will cover the sprectrum of personality types. If it's an environment you would choose to go to of your own accord, you'll probably enjoy it. If it's an environment that grates, then you likely won't find it so worthwhile.