To Series or Not to Series

So I just finished the LAST (I pray to the book GODS) book in a series. It was the SIXTH one. No it's not HP and I won't mention the name because I may be a little snarky in this post.

I know your asking "then why did you hang on for six books". CUZ I'm a hopeless romantic and optimist. I wanted to give the author one more chance to give me closure. I want the hero to ride through on his white stallion and save the day. I want the heroine to realize she is with the one she wants. I want the villain to die a painful death. What I got was an urge to stick a fork in my eye. At the end there was no real resolution. I didn't find out what happened to most of the main characters and the ending was the definition of anti-climatic.

I like a good 3 novel series. It might be that they feel like a whole story with a beginning, middle and end. It just feels right. Now don't get me wrong I love stand alones too. I love the ability to sit down with one book and know before I start I won't have to wait 6 months to a year to see what happens next. Sometimes I wonder if you lose readers in that time frame. Do they move on to other books and forget you have a sequel coming out? Do some readers not buy a series because of the wait for the sequels?

Do you prefer to read a series or stand alones? Do you plan on writing a series or are you a stand alone kind of writer?