I didn't really like the 5D1 while I had one, mostly for its lack of modern features. First a little background. My first modern Canon was the 50D, so that set my benchmarks in handling and performance. It was redundant when I got a 7D, so I traded in the 50D for a 5D1 to get the bigger sensor.
It was the little things that annoyed me about the 5D1. Each one not that big, but they soon added up. There was no usable auto-ISO mode, which was a big step back for the way I shoot. Especially as it handled noise quite well so it was not important I keep to a low ISO most of the time. That was the other thing. At ISO3200, it does look cleaner than the 50D did, and I actually wish they allowed even higher ISO settings. The screen on the back was a joke, so perhaps it was just as well the lack of live view meant I didn't need it as much anyway. And finally, it just never felt that sharp. I know the difference between 15MP and 12MP isn't that great, so maybe there's another reason for it, perhaps the AA filter is relatively strong? I don't know... and lack of AFMA meant I couldn't use AF with some primes I had. So again, all these points are not major in themselves, but they added up for me and I didn't keep the camera for long.
On the positive side, I have no complaints on the AF which at a practical level I could use as I did the 50D. As said I have no complaints about the noise at any of the ISO settings offered. And of course, the main reason I got it in the first place, the big fat sensor does offer the shallow DoF potential you just can't get on a crop sensor.
Only recently did I revisit this by buying the 5D2. It does resolve all the issues I had previously, and to me is worth paying the extra for.