Not full frame but not as much crop as the likes of a 600D etc. It's a 1.3x crop ratio rather then the usual 1.6x.
I see you own a 5D mk1 so compared to that, you probably won't be too impressed.
Though back when I had a Nikon D3 and a few pro f2.8 zooms I did get an offer I coudn't refuse on a 1D mk1, I borrowed a nifty fifty and was pretty impressed with what a 4mp camera could do. I never tried to print from it and can't say I ever used it at the higher iso settings but for normal day to day use the image quality was pretty stunning.
The only benefit you'd get over your 5D would be build quality, the 1D series are all built like tanks.
I sold my 1D mk1 on pretty quickly purely because I was set up with Nikon kit but since then I've made the jump to Canon and I have owned a 1D mk2 and a few 5D mk1s, I was sitting with a 1Dmk2 and 2 5Dmk1s a wee while ago and had to choose what to sell so did some high iso tests and ended up selling the 1D2 as it just couldn't keep up with the 5D. Sometime I miss having a bombproof camera though.
Have to say I was less impressed with what the 1D mk2 did as I was with the 1D mk1, maybe just because the mk2 cost me 3 times as much as the mk1 and it wasn't nearly 3 times the camera.