I would strongly suggest getting them, particularly right now. Have had season tickets for a decade now. Will never give them up. Celtics are a first class organization and they truly value their customers.
That and the product is improving by the second.
Agreed. I've had them since the 2008 playoffs.
To briefly answer your questions...
I do not believe there is a waiting list, and their sales team is actively recruiting new "season ticket members."
I enjoy being a season ticket holder very much. Honestly, I'm just a huge fan and it's nice to have the same seat for every game. Some of the benefits include access to high five kids tunnel, message on the jumbotron, potential shootaround on the parquet, preseason open practice tickets, and more, which all varies depending on your tenure and where your seats are.
According to the team website, season tickets start at $20 next year and go up to $280:
http://www.nba.com/celtics/tickets/2015-16-stm