The only thing making me have second thoughts about having this camera and lens is that they took a huge chunk out of my bank account.
Casey,
Obviously, from a financial standpoint, only you can decide whether it's a viable option to go for the 1DS. I was in your shoes many, many years ago ! And I can remember spending money on things like cameras and motorcycles and having that feeling of "should I really being buying this?" For me, it came down to the fact that if it was really my passion ( both bikes and cameras were...and remain so to this day... I'm 57 ) then I was willing to eat peanut butter sandwiches for lunch everyday in order to justify the money on the toys.
What I can tell you is that once you have used the one series camera, it's very hard to go back. Make no mistake, you can create excellent images with a Rebel or a xxD....but once you've shot with a one series, you will likely find you just don't want to pick up the other cameras. The only exception to that is if the weight bothers you. For me, the weight is a non issue....so after I bought the 1DS and the 1D2, I sold off my 50D and the two 20D's.
Overall, I'd say if you are blowing your entire budget on a body, and if it's likely that you will not have any money to budget for other things you would need for a long time, then it may be worth reconsidering.