In the last six years or so I've moved from a 300D to a 20D to a 5D, then back to a 40D in search of a balance between image detail, features and shooting speed. Image size was never really an issue as I'm not a pro, and the 50D just wasn't a big enough step over the 40D to make me upgrade a year ago. I love the 40D, it was the most "sorted" camera I'd ever used. So when the 7D came out I agonised over it and tested it a few times in shops - with mixed results since its version of RAW wasn't supported by Photoshop or DXO at the time. Then in a moment of madness I made the jump. And I'm VERY glad I did.
18 megapixels isn't where it's at for me with this camera and lots of tests have shown that it's still not quite got the resolving power of the full-frame 5DmkII - but then I've hardly taken a shot at more than medium image size or mRAW in the month I've owned it. What's really brilliant is the redesigned focusing and metering system, the unbelievable frame rate, the battery life (and a menu with information about the battery), the hi-res screen and real-world application of live-view tech that takes everyday photography to a completely new level (eg, held at arms length above your head). Also just how well balanced it feels in the hand and, I never thought I'd say this about a digital SLR: the movie mode. You might only use it once a month, and it sucks up a ridiculous amount of disk space but but it blows away handycams I've used for image quality.
There are a few minor downsides: for my liking JPGs are a bit soft, but I've ramped sharpness up in user settings and there is an "instant RAW" button that allows you to shoot an emergency duplicate RAW file without fiddling in the menus or having it on all the time with associated workflow problems. Also, in truth up to even large standard print sizes you'd be genuinely hard pressed to tell the difference between this and a good 40D image (or indeed a 20D image), but getting a good image is easier thanks to better metering and focus, and it's atarting to become more intuitive as I've got used to the camera. However if you use higher ISOs, pixel peep or crop images the difference in resolution becomes very noticable.
To sum up, the 7D is a genuine upgrade. Not just more megapixels, fancier menus and other augmented bits and pieces. It looks almost the same as a 40D/50D, and it still feels comfortably familiar as a consequence, but it is a new camera in almost all areas and a big leap forward for upgraders. Of course technology is no substitute for technique and there are many other DSLRs that will deliver stunning results at a fraction of the cost, new or second hand. But for die-hard amateurs like myself (or maybe canny pros) who have been on a Canon mid-range DSLR journey of discovery, this is the promised land.
Update 27 March 2010: Now been using the 7D for 5 months. I really like little electronic processing touches like auto lighting optimisation and lens-specific vignetting compensation that recognises my Canon lenses. Also, three custom dial settings let you store presets for certain situations - effectively replicating (with your own tweaks) some of the auto settings on the 40D. So I'm using C1 to dial in immediate shutter priority with AI Servo, centre-point focusing, partial metering, and full auto ISO for action shots. C2 is set up allowing me to achieve flash subject and background illumination in aperture priority mode by overriding the higher speed flash sync I've set as standard in custom functions. Finally I've set C3 to give me access to ISO expansion without fiddling with the menus. Speaking of ISO, since auto now goes up to 3200 with pretty good results I've been using manual and aperture priority more and more, letting the ISO do the work while I get DOF and shutter speed just right. Not one for 35mm purists obviously - but it proves the 7D's flexibility. Sadly, full resolution HD movies defeat my laptop and Adobe Premiere though they stream from the camera OK, but downsizing the resolution works fine (and uses less disc space). Still loving it!