My requirements here are:

• focal length range ~100 to 400mm in 35mm equivalent terms
• high image quality
• reasonably light and compact – < 2kg for the camera and lens and preferably < 24cm long mounted so it fits in a medium size shoulder bag
• image stabilised
• aperture at least f/4.5 to allow reasonable background blurring
• circular aperture blades for nice bokeh
• preferably sharp enough and with minimal aberrations to allow reasonable use of teleconverters
• close focus
• ultrasonic motor or equivalent for fast AF
• preferably weatherproofed so you don’t need to panic if it starts to rain
• medium priced

NOTE: ONLY the Olympus ZD 50-200mm kits satisfy ALL the above options.

Nikon D700 full frame with Nikkor 80-400mm f/4.5-5.6 VR:

• unfortunately the aperture is a bit slow negating use of teleconverters, etc and it is an expensive lens and is 1.36kg for the lens alone, while close focus is a poor 2.3m
• a pro would probably go for the Nikkor 200-400mm f/4 VR but that is much more expensive and tips the scales at almost 3.3kg!
• verdict – great high ISO and action performance but not for walkabout

Nikon D300 cropped sensor with Nikkor 70-300mm f/4.5-5.6 VR:

• gives 105-450mm range but at a slow f/5.6
• a nicer option would be the much more expensive and heavier Nikkor 70-200mm f/2.8 VR which gives 105-300mm and could be used with a TC

Canon 5DMII with EF 100-400mm f/4.5-5.6 L IS:

• again a rather slow aperture and I suspect its resolution will not be adequate for the 21mp sensor, and I’m not a big fan of the push-pull zoom mechanism while at 1.36kg its still a touch heavy and close focus of 1.8m is still nothing to write home about.
• I suspect it’s time for Canon to update this lens
• an alternative is the more expensive but much lighter 70-300mm f/4-5.6 DO IS lens
• verdict: not a great option for this focal range as a walkaround

Canon 50D with Tamron AF18-270mm F/3.5-6.3 VC lens:

• an incredible 15x zoom range giving 28-419mm range but lots of compromises on image quality and AF speed
• verdict: may be an option if image quality is not highest priority and don’t need it for action photography

Canon 50D with EF 70-300mm f/4.5-5.6 IS:

• 112-480mm range but again, slow aperture and resolution will not match the 15mp sensor

Olympus E520/E510 with ZD 50-200mm f/2.8-3.5 SWD:

• the lightest (<1.8kg for camera and lens), and almost the cheapest option, and provides some great features which beat all the other options:
• lens is weatherproofed (camera can be too if you buy the E-3)
• nice wide aperture with circular diaphragm for nice blurred backgrounds
• close focus to 1.2m giving 0.42x macro
• ability to use with either 1.4x or 2x TC with AF functioning and good image quality, allowing an incredible 800mm f/7 capability which can be hand held at 1/250th sec
• now if Olympus could make it a touch smaller and lighter, and work for Micro Four Thirds in contrast AF mode, it would be perfect
• verdict: MY favourite telephoto zoom lens – if you need action use, then consider the E-30 or E-3 cameras which will give faster AF, more AF sensor points for continuous AF and 5fps burst rates

Olympus E510/520 with ZD 70-300mm f/4-5.6:

• a cheaper, lighter alternative but at cost of image quality, slower AF and teleconverter use not recommended
• gives an incredible 140-600mm range image stabilised and not as obtrusive as the other options

If you really want super compact but high quality and no need for fast AF for action, then consider:

• Micro Four Thirds camera with forthcoming 45~200mm f/4-5.6 OIS lens which gives 90-400mm range in a very compact outfit, and I’m guessing there will be some continuous AF video-enabled camera bodies coming soon

It’s not just the camera to consider when buying a new camera but what lenses are available for it and how you want to use it.

I have a comparison table of lens features here, some example photos I have taken with this lens, and example photos taken with this lens with Olympus 2x teleconverter (EC-20).

Bob Atkins has a comparison of entry level dSLRs here – although he does not go into length on the relative benefits of the lenses available to each.

Brandon Eu has a nice blog demonstrating how useful this lens is.

In my opinion, for the amateur who can afford it, the Olympus ZD 50-200mm SWD, the excellent Olympus teleconverters and general high optical quality of their lenses are some of the main reasons to choose the Olympus system, the cameras are rather secondary.