This isn't one of those "which should I buy?" threads! I've got both bodies now.
The 7D is my camera. I just picked up a used 1ds as a backup/second body for a trip to Yellowstone. Figure I'll keep my supertele on the 7D for wildlife and have either my 200L or 35L on the 1ds for closer encounters and/or portraits and landscape. (I'll probably end up selling the 1ds after the trip unless I fall in love with it.)
I just wanted to say WOW. I'd never used a full-frame camera before and I can see now what a difference it makes! Because I do primarily wildlife, I don't see myself switching to FF permanently, but I get it now.
First of all, losing that 1.6x crop extension is a big deal! It's like I've suddenly doubled the size of my kit! I can't believe how wide the 35L really is, as I've only ever used it on a crop sensor.
The other big difference that I was less prepared for (even though I'd read about it) is the difference in depth-of-field. The 35L is f/1.4, which is damn shallow even on a crop body, but holy cow is it shallow on the 1ds!
I can see now that switching from crop to FF involves a steep learning curve. I can also see the allure of having both a crop and FF body in your kit as a way to gain versatility without having to invest in lots of new glass.
Just thought I'd share.