I get gameplan. It's awesome for watching lots of football, but you won't get too many big 12 games on it. Maybe one or two. It depends on where you live.
What you get is essentially the out of market ESPN/ABC games, and a few that are carried on some of the regional networks in the south. What you don't get is the other regional FSN games (you would need a separate regional sports package for that), or some of the locally broadcast regional games (think mid-majors).
The BTN has sort of obviated the need for much of it, IMO. Last year there were a lot fewer games of interest on it, since the BTN had 2 or 3 games per week on it that used to be on Gameplan.
I'd say it's worth it, but only if you're a pretty big fan of college football in general, as opposed to just wanting to catch a few ISU games (which is quite unlikely this year).