I have a Canon 40D that I've been very happy with, but some of it's limitations were starting to bug me enough to upgrade:
- Poor low light/high ISO performance (3200 is noisy)
- Not full frame (always feels like I'm wasting my L lenses)
- No video (no big deal, but it's nice to have)
- Less than ideal battery life (for a full day of shooting)
- LCD could be better (usually only a problem in sunlight)
With the modern camera line-up, there are stacks of different ways to jump. I looked at getting faster lenses (I love my Nikon 55mm f/1.2 MF) for better low-light performance - I examined my most used focal lengths, and worked out a cost of getting 1.4 or 1.8 lenses in those. That was about the price of a 7D, and that only solved one problem.
Simarly, a quick trip to DxOMark quickly told me that the 7D wasn't worth the upgrade cost over the 40D - the only problem it solved was the lack of video.
The kings of low-light performance in the Canon range are the 1D Mk IV, 5D Mk II and the 1Ds Mk III, but the 1Ds Mk III is the king of wallet damage as well, which ruled that out as a viable option, which left the 1D and the 5D.
The 5DMkII is the choice of wedding photographers for it's combination of a reasonable price, with full frame capabilities and great low light performance.
But, the 5D, like so many Canon cameras (!!?) suffers from pixel stuffing - how high can we bump this megapixel count? Nikon got sick of that game a while ago. Not many people need that kind of ridiculous pixel count, and it just means you have a lower frame rate (3-4 pics/second - yeah, the 40D doubles this), and the pictures take more time to unload, process, etc.
It also suffers from an older AF system (compared to the 7D & 1D), a slow flash sync speed (a worry for a strobist), and the battery life is less than on the 40D!
All annoying downsides of a substantial 'upgrade'.
It's low light performance is bordering on insane. While the 5D may have a slight edge on quality (if you believe DxOMark. Or if you look at the right sample images, the 1D has about a 1 1/3 stop advantage over the 5d), the 1D goes a further (usable) stop of ISO, just incase you HAVE to get a picture.
As far as battery life goes, one charge gets you about 50% more than the 40D, and it gets a tick for it's great LCD (no, really, it's stunning).
Video is included, so the biggest downside...?
It's big; it's heavy? So am I. It's not full frame? But it's closer than the 40D is.
Ok, it's bloody expensive. But if there's one thing I've learnt from the Strobist... rather than upgrading every couple of years, getting the best could mean no upgrading for a lot longer. The cost is spread out over a longer period.
At least that's what I told myself...
(Incidently, the Nikon