To me, it's worth the dough.
You get access to tons of videos that are subscriber-exclusive (they currently offer 2-3 minutes of pretty nice quality video of each corps's performance at Lisle, Ill. As the season progresses, more videos like these will be added).
There are heaps of audio, too. You get interviews with corps members and directors, as well as reduced prices on the APD's (might not seem like much but if you buy many of them, the discount stacks up pretty nicely). Season Pass also features the "Field Pass" audio magazine, which is usually pretty interesting. The feature that I use the most is the FREE unlimited streaming of any Top 12 show since '72.
I just renewed my subsciption a few days ago (no real point to have it from Sept-May; they say they continue to add stuff but it's not really worth it, if you ask me) and am very welcoming of the stuff I missed when I didn't have access.