After shooting over a 1000 images in a few hours I can share my first impressions.
One benefit after only just buying it, was that it was very easy to "get to know". If you are familiar with Canons then it has most things in the usual places. A few differences in layout but a long-time Canon user would be comfortable with it very quickly. Testing on Wednesday and Thursday established that the image quality was excellent. I did lots of low-light tests, in preparation for the wedding and established that it was excellent in marginal light at high ISO's. This was good since I forgot to turn on my flashgun in one very dark environment and all the pictures were taken in very dim natural light. The performance at 3200 ISO was excellent, fortunately for me!
Having had the 550D for a while I was expecting this. The results are really quite remarkable for a cropped 1.6x sensor. Whatever you say about Canon, and I've personally said quite a lot!, their ability to create wonderful sensors with low noise at high ISO's and sharp detailed resolution is second to none. All this with 18MP is remarkable. With its weather sealing and sturdy construction it is very much a "pro-spec" camera at a non pro. price. This must be the best APS-C sensor camera so far.
Its blindingly fast, for focusing, shooting and playback. The motordrive at 8fps is incredible and not that noisy either.
I liked working with it better than the 5DMKII I had. I find it handles better and the whole thing feels a step up from that camera. The 5DM2 still wins in the low noise stakes, but not by much. I need to test more, but my suspicion is that the 7D might be better at low ISO's. The cropped sensor suits my Canon lens collection more than full-frame and gives me a depth-of-field advantage by effectively increasing the focal length of my lenses. For some reason, even though its slightly heavier than the 5DM2, it doesn't feel it. I have no idea as to why that should be the case, but carrying it around for several hours didn't seem a burden.
On the minus side I've got the usual CA and purple fringing problem that I always seem to get with Canons, but its mostly fixable and liveable with. While it will never win a camera beauty contest it does seem to be reasonably aesthetic for a Canon. The battery grip looks like it might have actually been designed with the camera in mind, rather than Canons usual somewhat functional approach to these items.
All in all I'm rather pleased with it. I'm looking forward to checking out the video facility, which promises to be excellent and I'll see how long I can cope using it on a landscape trip. I'm actually quite looking forward to that, which considering its weight and size, compared to what I've been using recently, is proof that I'm quite taken with it. It was bought as a solid, sturdy high-spec. work camera but at the moment its attempting to persuade me to use it for other less stressful excursions. We'll see how that goes!
I don't usually make recommendations but if you're a Canon user looking to upgrade, or are looking for a good all round camera at a decent price then I think you would find it difficult to be disappointed with the 7D.