As to keeping up with the Joneses, that's exactly what I'm trying to avoid. While the 5D MkII sounds like an enticing camera, I'm not convinced it's a good upgrade for its price.
The cost of using a camera is the depreciation that you will incur by holding it and using it for a year.
When a new camera comes out, the correct comparison is between the depreciation you will incur on your existing camera during the coming year, vs. what you would incur on the new camera over the smae period.
For example, I bought my Canon 1DsII in January 2005 (I think). Bought it for $7,800. Sold it for $4,800 2 years later (I think - a bit fuzzy, but the numbers will serve as examples.) So the cost was $1,500 per year in depreciation.
The 1DsII is selling now for about $1,800 (rounded down a bit for ease.) So holding it the next 2 years,. from 2007-2009, would have cost $3K, again about $1,500 per year.
When the replacement 1DsIII came out, the real question would have been the depreciation I would have realized on the 1DsIII vs. the $1,500 on the 1DsII. Based on history the cost may have been about the same, so that upgrading would have a net incremental cost of $0.
If I had replaced the 1DsII with the 5D in January 2007, the cost would have been Purchase Price ($2,600) - Sales Price in 2009 ($1,000) = Net Cost ($1,600) / Holding Period (2 years), or $800 per year.
So Moving from the 1DsII to the 5D would have saved me $1,500-$800 = $700 per year. Net cost for owning these digital cameras would be:
$1,500 2005
$1,500 2006
$800 2007
$800 2008
The correct question, then, when a new camera comes out is: how would my depreciation over the next year compare with the new camera versus my existing camera?
With the 5DII, I imagine depreciation will run about the same as the 5D has, say $800 a year. Holding the 5D for another yaer, in comparison, might cost about $350 ($1,000 to $650.) So the real question is whether the upgrade is worth an incremental $800-$350= $450 Or if you already have a 1DsIII, selling might save you $1,500 - $800 = $700 a year.
Based on the historical prices of MFDB's, your depreciation cost is likely to have been much more than the $1,500 a year in the 2005-2006 time frame.