Hi,
I made the switch from EOS3 to 10D, and really enjoyed "going digital" right from the start. Friends of mine have made similar switches in Canon and other camera systems and told me they felt "reinvigorated" by all the freedoms digital photography offers. Some of those folks (serious amateurs, semi-pros and pros) had been shooting 35mm film since the late 1950s and early 1960s and were initially highly resistant to making the switch to digital. None have regretted it and switched back! That alone should tell you something.
The viewfinder on any 1.6X crop camera will be smaller and less manual focus friendly than on your film camera. The 40D addresses some of the compaints with a VF that is improved in several ways, including greater eye relief, slightly larger image and interchangeable focus screens (limited screens are available right now, but do include a high precision matte that will help with lenses f2.8 and faster).
However, I think you will be more dependent upon AF with a 40D than you have been with your EOS3s.
Digital images are in many respects better quality than even high resolution film scans can produce, if that's what you have been working with. I have a Nikon 4000 scanner and shot mostly Velvia 50. Nice, but I find digital images to be even cleaner and more grainless. Digital images "run out of gas" faster than film, when enlargements are done. With a 10MP camera like the 40D, you'll be able to do excellent 16x20, but enlarged much more and film can be better (there are softwares that can help a lot, though). I can tell you for sure that ISO 400 and 800 digital will easily beat any film on the market. 1600 shows some noise (instead of grain) but is still cleaner than film of equal speed.
When I bought it, my 10D and a few necessary accessories cost me more than *two* 40Ds would cost today. Still, that camera paid for itself in savings of film purchase and processing within less than a year. Now a shoot that nets me 1000-1500 images a day costs me nothing but memory space (granted the equipment will wear out and need to be replaced some day). The same day's shoot with film would have cost me $400 to $600 out of pocket before I saw a single image. Also, I can review the results in minutes, rather than waiting for 2 to 7 days for processed film to come back (slides).
So, I no longer feel restrained by the cost of film. Result, I shoot more images and take more risks... with great results. (I still go back and shoot film with both my EOS3s and several other classic systems during my "off season", because I think it's good for my shooting disciplines.)
It's also great that a 2GB CF card is good for about 200 shots, rather than having to change rolls every 36 exposures. It takes seconds to swap in another CF cards, and be ready to take another 200 shots!
Not to mention I can change ISO on every single shot, if need be, don't have to change film mid-roll when lighting conditions change radically.
The LCD screen allows you to review images, but is not as accurate as a computer monitor, so is best not used to pass final judgment on color and exposure. However, there is also the histogram, which, once you learn how to use it , will confirm exposures neatly.
And, I now carry just a few filters most of the time (3 sizes of UV, warming and polarizer, plus the same types in drop-ins for super teles), where I used to carry literally dozens. I can set the white balance to correct for odd light color issues. Or, I can just correct in post-processing, on my computer.
Finally, shooting RAW files is great. They allow lots of tweaking and refinement after the fact, more than JPEGs or film ever could. With Adobe Camera RAW and Adobe Bridge (in Photoshop CS2 in my case) I can open 40 to 50 image previews and do all sorts of pre-adjustments, then just run Image Processor and go get a cup of coffee while the software converts to JPEGs or TIFFs. Included in this is tweaking saturation up or down, leveling an errant horizon line, expanding or contracting dynamic ranging, changing white balance (color temperature), even marking images to trash or simply not use right now.
I don't yet have a 40D, but the AF on my 30D is close to the performance of my EOS3s (and much improved over the 10D). There are fewer focus points, but I usually restricted my EOS3s to 11 or 13 anyway (via CFn, with spot metering linked), so the 9 points the 30D uses aren't all that different for me. In fact, they reach more toward the top, bottom and sides of the image area than the EOS3's oval of 45 points does. The 40D has apparently seen a number of speed and sensitivity improvements in AF, compared to the 30D. All I've heard to date sounds darned good, although perhaps not quite as blazing fast as the 1D Mk II N's (the jury is still out on the 1D Mk III's AF, as far as I can tell).
If your budget allows, get a Tokina 12-24/4 (about $500) or a Canon 10-22 (about $650, if memory serves). One of those will get you the wide end of things back, although they are really only usable on the 1.6X crop cameras.
A lot of folks overlook "the other stuff" when thinking of converting to digital. Allow something in your budget for additional computer RAM, hard disk storage space, a solid backup system (extra hard disks and/or DVD burner), color calibration for your monitor (at least... printer too if possible), post-processing software (Photoshop or similar), and books and/or classes to learn how to use everything. (The DAM Book: Digital Asset Management for Photographers, and the Magic Lantern Guide for any particular camera are both highly recommended.) Another very nice thing to have is a graphics quality monitor. There are both CRT and flat screen, the latter being much more expensive for truly graphic quality. And, you'll want a photo quality printer of some sort, in all likelihood, if you don't already have one.
It's really too early to say if the 40D is going to be as reliable as the 10/20/30D have been, but I would expect so. FIrst userreports are that it's a great little camera, and the first reviews seem to predict a winner. Only time will tell if that's true, or we're just in a honeymoon period with ga ga new owners. But, it's not a camera that really tries to stretch the envelope out close to the breaking point and just has a bunch of reasonable improvements and upgrades from its predecessors. My 10D and 30D get regular, hard work outs and have given me very little trouble (3 years with the 10D, just over 1 year with the 30D).
By the way, most sports events I have EF 24-70/2.8 on one camera, EF 70-200/2.8 IS on the other. A great combo for me! However, I will eventually add a 12-24 to replace 17-35/2.8, if I stick with the 1.6X cameras (the 40D is very tempting!). For less hectic situations that allow it, I tend to go with prime lenses: currently from 20mm to 500mm, plus 1.4X and 2X.
If in doubt, perhaps you could rent a 30D (I doubt any 40Ds will be avail. for rent yet) and give it a try.
If you have ever used a digital P&S camera, the images from a D-SLR will blow you away. They are much cleaner and nicer. A 10MP P&S will almost always have as much or more noise at 200 ISO than a 10MP D-SLR at 800 ISO. The sensors in digital P&S cameras are really tiny, which makes for more problems with noise, especially as competition drives the MP count higher and higher. Larger D-SLR sensors, even in 1.6X crop cameras like the 10/20/30/40D, will give much cleaner images and more enlargement potential. FF digital can take it even farther, of course.
Have fun shopping!
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