I have had many cameras in my day. However, I got into micro four thirds by accident. Thanks to a very helpful Olympus agent at John Lewis and a similarly helpful Panasonic agent at Panasonic, I was able to experiment with cameras and lenses.
If you shoot in daylight, believe me - any camera is good. The distinction in performance comes with low light conditions and sports etc. If it's daylight you shoot in, you can even use your mobile for great results. However we live in a country that is darker more than it is lighter, or overcast and raining. True, you can use a flash in any situation and in portrait photography lighting is of the essence. But wanting a good camera that shoots in low light without the need for a flash, with little noise.. (all cameras can shoot in low light but the noise levels are extreme to make the photos grainy etc.) .. you should go for the E-PL3.
Compared to its sister micro four thirds camera, the Panasonic G3, the Olympus PEN E-PL3 is the better camera, personally. True, if you went online and read the reviews, the G3 trumps. It has a touch screen, a built in flash and an electronic viewfinder. You get a free flash attachment, no touch screen and no electronic viewfinder for the Olympus. What you get, however, is a very good camera if using it WITHOUT a flash and in low light conditions especially.
The electronic viewfinder for the Olympus, the VF-2, costs £176 at least.
I have shot pictures with both Olympus and Panasonic lenses on both the EPL-3 and the G3 Lumix.
The EPL-3 works well with the Panasonic LEICA 25mm F1.4 lens and the OLYMPUS M.14-42 F3.5-5.6 II R lens.
The ZOOM OLYMPUS 40-150mm lens needs a tripod, despite the image stabilisation built into the body of the EPL-3.
The Panasonic 14-42 F3.5-5.6 lens is reasonable but a better alternative is the Olympus 14-42 lens.
The Panasonic does not have lens image stabilisation, whereas the Olympus has image stabilisation built into the body of the camera.
The Panasonic has lens image stabilisation in the 14-140mm zoom (Panasonic) and NOT the 14-42 kit lens or the 25mm prime.
So, yes there are advantages and disadvantages depending on what you see as relevant features to yourself.
I wanted a camera that would perform well indoors without a flash to capture beautiful shots, not spoiled by the glare and intensity of a white flash.
The M.ZUIKO lens are actually affordable. Go for the normal lenses, and the minimum you would pay for is at least a £1000.
The beauty of the Olympus is the built in image stabilisation to the body, and NOT the lens. This is a feature of high end DSLRs. Even Nikon entry level DSLRs have image stabilisation built into the lens rather than the body - so called VR or vibration reduction.
The olympus kit lens 14-42 is thinner, smaller and lighter than its panasonic counterpart. Even the 40-150mm zoom lens is the same. Have you seen the Panasonic equivalent. It is usually sold with the Panasonic GH2 as a kit lens, and by golly is it heavy.
Unfortunately don't expect a lens hood with Olympus, you have to pay extra for that. Panasonic gives you the lens hood with its lenses.
The reason I went for the Leica 25mm F1.4 lens (prime, from Panasonic) is because the F1.4 (very low) is perfect for low light. If F1=the human eye, than F1.4 is closer to a dilated eye letting you see on camera what you'd see in person. The lowest focal length of 0.67 goes to the Phase One cameras which costs £18000 pounds with perhaps £3000 for the lens!! Yes, you'll spend £700 on the Panasonic F1.4 lens but it is a bargain.
Micro Four Thirds cameras and lenses are expensive. You could easily buy a good DSLR, but you pay this money for smaller, compact cameras with lightweight lenses that you can use just like a compact for point and shoot photography. If you have a six year old like I do, lugging a DSLR and running after a kid are inconceivable. Owning the EPL-3 with its lightweight lens means you can run after your kid without worrying about the camera. It fits into your hand.
Do be mindful that you do not get the 'grip' you would with DSLRs and even single lens zoom cameras. Otherwise go for the Panasonic FZ35 for e.g. In fact you could easily buy an EOS or Nikon DSLR for what you will spend on a micro four thirds!!