I recently sold my Canon 20D, which I felt was a good move while it still had some value. Since then, I've been hired for a handful of photography gigs (a wedding and a couple of headshots) and am left with just my 1D Mark II N.
There is absolutely no way I will do any paid shooting without a backup camera on hand. The possibility of camera failure when someone is paying for results is just not something I can risk. Plus, there is a benefit to having two cameras with different lenses ready to go at all times.
I checked into rental rates for a backup body and was looking at about $150 a day. I couldn't bare the thought of paying that much per job and taking it out of my profits, so I invested in a Canon 5D of my own. It was a big decision, but I know it will pay off in the long run and retain its resale value when I upgrade in a year or so.
Between the 5D and 1D Mark II N with my arsenal of lenses, I have everything I need to cover most situations. Plus, the 5D gives me some things I haven't had before, namely, 12.8 megapixels and a full-frame sensor. The higher resolution will be perfect for jobs that require large prints, and the full-frame sensor lets me take full advantage of my wide angle lenses. I threw on the 16-35mm lens, and while I've used it on a film body before, it's incredible to see such a wide view on a digital SLR.
I'm anxious to take it out for a test drive, but may not get the chance until Sunday or next weekend.