Which do you enjoy better, series (books, comics, movies, shows) that are still ongoing, or are done and over with?
It seems to me that the benefit of a finished series is that you know, based on reviews and hearsay, whether or not it's worth your time. If it's worthwhile, you have a whole lot of good stuff to look forward to, without any delays or substandard installments. You also run the risk of spoilers, but if it's really excellent, that shouldn't detract from the overall entertainment value. Maybe there's not as much mystery or suspense as an ongoing series, but disappointment is not as serious a problem.
The advantages of ongoing stuff are pretty much self-explanatory. It's not done yet, it could go on for a long time (for good or for ill), and everybody is on the same page, figuratively speaking (or literally, if it's books). There's a lot more room for argument and speculation. The logic of games and gaming eclipses storytelling. The narrative is just part of the experience; there's also the endless questions about what happens next. Discussing the ramifications of the plot with friends or coworkers or online acquaintances makes it into more of a communal experience. If a finished series is like a video you watch alone late at night, an ongoing series is like seeing a movie on opening night in a theater packed to capacity.
A confession: I tend not to follow a whole lot of series. I only watch a handful of TV shows, I don't read many comics (and tend to read them in collections), and there are only a few book series that I read regularly — Song of Ice and Fire, James S.A. Corey's Expanse series, and Richard Kadrey's Sandman Slim books. (I used to read a bunch of detective fiction, but most of those have fallen by the wayside over the years.) I think that's because I tend to get burned or bored pretty easily — I hated the ending of King's Dark Tower saga, and Battlestar Galactica is still dead to me.