My first DSLR was the ol'Canon D30 (purchased used when the 10D was the released)... It met all my basic needs at a good price and reliable. At the time, my interest was still with film (still is to an extent) but work and family were increasingly cutting into time spent with my camera. I wanted something that provided a bit more convenience without resorting to a P&S cheapo... the D30 filled that need wonderfully.
The combination of wanting more pixels to work with (cropping) and my friend's interest in the camera (She still uses the same camera to this day) convinced me to graduate to the 10D. The 10D was once again purchased used around 6 months after the release of the 20D. Again, I took advantage of rock bottom prices pushed by the general public's want for the latest and greatest. I was in heaven! To be honest... I could still be happy with this camera today.. There is absolutely nothing this camera was "preventing" me from being creative and enjoying my love for photography.
A year later, I discovered a photographer who was interested in moving to the 5D which was released 4 months earlier. After bargaining for a few minutes, I purchased both of his 1d-markII for a really low price. One was subsequently sold on
ebay which equated to the 1d-markII costing me $1500 (still an unbeatable price even now). At this time, I was still in the business of flipping camera equipment for some pocket change and intended to sell both for a good profit. I ended up keeping it for my self and selling my 10D to my cousin for $400 (I was being a nice relative).
BTW.. that photographer is a journalist and needed something reliable with a good build that was just not there in Canon's "prosumer" line (10D, 20D, 30D). He had the 1d-markII as the next best choice but hated the bulk and weight. When the 5D was released, it was right up his alley and his eagerness to fund the 2 5Ds for work gave me a lot of negotiating leverage. IMO, the 5D is one of Canon's best decisions and products to date.
I've been shooting the 1D-markII ever since.... I see absolutely no reason to upgrade at this time as it is more than I need. I will probably stay with the camera for years to come (unless I jump the Canon ship).
I think most consumers buy cameras like cars... they want the latest and greatest or just something new. MOST cameras out there are more than any of us really need or leverage. This is the main reason why one of the most common questions asked by shoppers "Is Canon releasing any new cameras soon?" They are concerned with having the latest and greatest for only a few months.... rather internalizing their purchase to their needs. This is all fine... whatever makes people happy... I"m sure just as guilty.
So how do I know I"m ready for an upgrade? In the case of ElanIIe film to D30, I needed a quicker workflow to enjoy what little time I had for photography. In the case of the 10D, I needed the ability to crop and produce 8x12 or 11x14 prints. In the case of the journalist, he needed a professional Canon without the bulk of the 1DMII.
Except for me acquiring the 1dMII simply because I got a hard to pass deal, I first won't upgrade until my current body prevents me from doing something. I also found that there is no good reason not to take advantage of used equipment.... especially when the next best body is released.
Sorry for rambling... just felt like typing this morning...