Right now I have a complex rotation going due to this very problem. I just float between doing the FIRM, Jari Love, Jane Fonda, Jillian Michaels, Leslie Sansone, Jessica Smith, Ellen Barrett, and the few yoga videos I have, and I made myself a little list where I rotate the type of workout for the day and then have some add-ons from 10 Minute Solution and the FIRM's 5-Day Abs videos. I have so many FIRM DVDs that I literally do them in chronological order and have them set up that way in a big dresser. I want to discourage myself from overthinking what I feel like doing, so I've been sticking to this "anti-rotation" for about three months now, and I like it. I also go through month-long mini rotations, like just Jari Love, just pink FIRMs, just BeMax, etc., but I NEED variety, and I think my body responds better to muscle confusion.
I tried to do FIRM Express as prescribed back when it first came out, and I got soooooo bored. So I feel ya. I think if I ever did P90X or similar I'd do it as prescribed once through and then just break the workouts up and put them in my anti-rotation.
That's how I currently treat FIRM Express and it has much less dread factor as a result.