i think those people who has only used 1 system (brand) shouldnt give comments at all. cos in the first place, its like telling me your apple taste better than the orange because the hardware specifications say so.. you havent even tried the orange before.
Here's my take based on my personal experiences on the body used so far personally.
(1) Canon 300D(sold), Nikon D100 (sold) , olympus e-300 (still currently using), minolta 5d (sold) , Sony Alpha A100 ( still currently Using)
I'm not going to have a comprehensive review on which systems is better for you. But i'll elaborate why i decided to switch systems and why i decided to stay with them.. You decide on your own which system you prefer.
Canon 300D with various AF lens + carl zeiss/takumar manual lens >> 300, 350, 400D are the "consumer" series of the canon DSLR. what do i meant by consumer ? , the body is plastiky although its light.. lesser features and functionality. Lesser buffer. User friendly interface, easy-to-learn for beginners. Summary : value for $, no frills, easy to resale, easy to buy 2nd hand lens. However for really quality lens, "L" lens or "IS" lens, you need to pay more. Nice saturated colours, photos produced in good sunlight are "colourful". You either like it or you dont.
Nikon : For the same comparison for example , D70 comparing to 300D, D70s Comparing to Canon 350D, the latest D80 comparing to CaNON 400d. nIKON WILL always be slightly higher priced than canon competitive series. why so ?? Nikon strategy seems to be on building more robust body and handling comparing to canon. Nikon body really has lotsa buttons, especially i used the D100. its amazing, comparing to EOS 20D. Image quality is different, colour tones straight from the nikon, canon are always different. i can't tell you which is better becos its very subjective. Summary : You hold the nikon body, it feel sturdier, robust. However it doesnt meant its better than the canon competitive series, you pay more $$ for a nikon, thats what you deserved. Re-sale value is also high, quality VR lens are a fortune, similar to "L" lens. , 2nd hand lens easy to find and buy.
Minolta 5D , Sony A100 : The main reason for anyone to go into this brand is the Image stablizer. Image stabalizers are useful for beginners cos it has higher tolerance for mistakes .. Meaning in a average day shot, you end up with lesser blur photos due to handshake especially for shots handheld and 50mm and above. And any lousy lens on the body has image stablizing effect. Thus Sony A100 with a kit lens 18-70mm F3.5-5.5 and a canon 350 D /400D 18-55mm kit lens.. i would say after a full day shots, given to a beginner, you will see that shots from Sony are sharper due to some situations the Anti-shake thing kicks in.
For experienced photographers with loads of $$, there are of course Nikon VR lens and Canon IS lens to help you with the low shutter speed,"shaking" problem and also Canon and nikon really has better ISO noise control so you can always shoot at higher ISO comparing to the Sony A100. However the A100 for 10 Megapixel, Anti-shake, anti-dust, Dynamic range (althought its not useful unless you have a bag of lens and change lens so often) and selling for $1599 is pretty amazingly cheap. Construction wise, sony a100 is rather plasticky not as good as nikpn, maybe closer to canon 400D. thats about it.. resale value not good, good minolta lens hard to find. I've owned "G" lens that cost $1000 a piece and it produces great results. However, its not easy to find on 2nd hand market and it gonna cost you a bigger bomb than nikon Vr or canon L lens. Only advantage >> Anti-shake on body even with every crap lens mounted.
Olympus >> currently still using the e-300, which main reasons : Zuiko lens and the colour tones the olympus system it produces. I wouldnt go into detail what 4:3 systems is and the advantage or disadvantages. Olympus is a rare and niche group. Its like driving a volkwagon whilst most people will say toyota, mazda and honda are the best. For beginners, good zuiko lens will make you "feel cheated" cos its damn expensive comparing to other brands. However, i've used 4 systems before. I would dare to say, in terms of colours and bokeh, especially coming to skin tones, be it outdoor or studio, olympus is close to my Carl zeiss. The "live-view" feature on the new e-330 or e500 is a feature where you either love it or hate it . Live-view can be useful for beginners whom are switch over from the Point and shoot cameras because you can frame picture on the LCD before you shoot. Thats about it. Some people use live-view to take difficult / constraint positions and angles. I dont think it really appeal to professional, it didnt appeal to me.
Summary : You might not appreciate olympus when you just started photography. The lack of VR/IS/ Anti-shake on olympus might be a drawback and good F2.8 lens costs a bomb. It produces great colours, tonal range and picture quality has a special feel. Body unless its the E-1 , their construction is "so so" better than my sony A100, and canon 300d/400D, but not better than Nikon D100 or D80. Go try the live-view feature, that could be only main reason for beginner to buy it