That's great for the people who've been reading since the start of those fifteen years. Not so good for the kid wanting to jump in somewhere in the middle of year eight.
Problem is that the latter invariably outnumber the former.
The thing is that it was a continuous story - more like a soap opera than an "arc" show like Babylon 5. It was fairly easy to jump on and figure out what was going on, with plenty of exposition to cover the bits that needed to be covered.
For example, in 1981 there was a two-parter X-Men story published called Days of Future Past (the movie wasn't an exact copy, but shared some of the same concepts). It showed a dystopian future where Sentinels had taken over America, and mutants were essentially kept in concentration camps. This future came to pass because of the assassination of a senator championing anti-mutant legislation. However, the remaining X-Men succeed in a plot to jam their power dampeners, which allows Rachel Summers, a young telepath, to project the adult Kate Pryde's mind into her past self, Kitty Pryde. In the past, Kate/Kitty gets the X-Men to stop the assassination, but instead the senator's wife dies. That's a change, but not exactly the rousing success they hoped for.
However, three years later (in real time), Rachel Summers manages to travel through time herself (instead of just sending Kate's mind back), and thus enters the "mainstream" Marvel Universe. In these issues, the background is explained so you understand what's going on, but if you've read the original stuff it's a much deeper experience. And then, of course, while the team are having all sorts of other adventures, Rachel has to deal with being in a world where her mother died as the Phoenix instead of the more peaceful resolution in her time-line that eventually lead to her birth, and all sorts of stuff like that.
I think the secret sauce was that in most cases, the long-term plot development was handled like B-plots, with more immediate A-plots to get resolved. There's a bad guy who wants to do bad stuff, punching ensues, and in the background there's some character development happening. The reader gets the immediate pay-off from the punching bit, but is also enriched by the character development. Now, the A-plot takes six issues instead of one or two to resolve, and there pretty much isn't any B-plot.
