Phil Askey, the founder of DPReview, commented that, for many 5D buyers, it "completed the transition from film to digital." Certainly that was the case with me. After I bought it, I only shot one roll of film, in an EOS3 that I loved. It wasn't until I bought a 5D3 that I would have a camera as good as that EOS3.
I went straight from Minolta Dynax to the 5D. Passed on the 5D2 and eventually bought the 5D3. Sold the 5D only a few years back to a first time full-frame user and he was stoked. Much like I was when I first bought it. Like the early Leica digitals, had a certain look to the files which the later (higher mp) sensors didn't.
I had owned the original Digital Rebel, added an XTi when it came out, retired the Rebel for a 40D and still felt that something was missing (prior to digital, 34 years of 24x36mm film cameras, mainly the Canon F-1). I owned the (for then) brilliant EF 70-200 f:2.8 L IS, but of course on crop it ends up as 110-320mm, not ideal for portrait. The 5D changed everything. Finally a proper use of that lens' capabilities, and the 24-105 L that came with the kit was the perfect flash studio lens (for me). The inexpensive 17-40 completed the camera bag, with a 35 L for low light. I have had on and off some APS-Cs for specific uses (7D for sports, SL1 for lightweight) but full format was the way to go and the 5D opened that door. I purchased a second one to reduce lens changing and have backup on long travel expeditions.