In general, I don't think it's a valid argument to say that any one brand or particular make of DSLR is the clear winner over all others. If that were the case, we would have an iPod situation (the original iPod) where 90% of the market goes to that model. Truth is, any DSLR will take good pictures, and any DSLR can take crappy pictures.
The advantages for the e620 *for me* are:
1. Light weight and compact size. This is important for me because I do a lot of hiking and take trips with my camera, carrying it all over cities and in the wilderness. Weight and bulk make for un-fun excursions. No, it doesn't fit in your pocket -- unless of course you're wearing hiking cargo pants and using the 25mm pancake.
But it really does make a big difference. I can carry my e620 body, 25mm prime, two kit lenses and even a fill flash in a small-ish camera bag on one shoulder, no need for a backpack or bigger stuff. I can of course strip that down with just the camera and a spare lens in one cargo pocket too.
2. *kit* lens quality is above par (not head-and-shoulders better but sharper across the focal plane) compared to other kit lenses out there, and the upgrade path to items like the 50mm prime, the 12-60mm zoom and the 50-200mm (these are in 4/3 terms) which truly are superior lenses makes this a smart move in the long term too.
3. I do think the bang-for-the-buck factor, the number of features and level of control of the e-series is a plus. The fact that, after setting up things maybe once or twice in the menus I can do 90% of what I want from the LCD control panel and/or dedicated buttons at this price point is great.
4. For landscape photography, the lenses that keep better focus at the edge of the frame and the 4/3 DOF are very well suited. That said, I've neither had problems with shallow DOF/blurring of the background in my macros and portrait shots, nor have I had big problems with noise so long as I expose well, and turn off auto-gradation for low light shooting. Faster possible lenses (which are expensive, mind you) make this is non-issue.
5. I like the very good JPEG processing that chooses detail even if it means a bit more grain (I like grain) or possibly noise (compared to Canon's engine) and keeps more natural colors for the most part (though you can argue about whether the infamous Olympus blue is natural). What I see is what I get usually, and I like it that way. Nikons are just as good at this IMO and their greens are a bit better.
6. I think the 4:3 image format gives me more flexibility when cropping. Even though I do a lot of landscapes, I prefer it to a 3:2 proportion which is more limiting, especially when you realize that a pic might be better as a portrait shot instead of landscape. I deally, a square or [sq. rt. of 2] rectangle are the ideal formats, the former because it's the most flexible, and the latter because it aligns with standard ISO paper sizes. More on that later.
Disadvantages I see are:
1. I think the menus are a bit of a liability because the wrench menus in particular are perhaps too thorough. On the other hand, you have access to so many features and controls, someone who knows what they're doing has a lot of power. Someone who doesn't can of course just turn off this option to even see those menus. Still, I think Nikon and Sony have better UIs in the menus.
2. For all I said about high ISO not being an issue, if you *are* off on your exposure, or forget to turn off auto-gradation, or if you are using the slower kit lenses, noise is more of a factor in your shooting. You have to be more mindful about it, I think.
3. [clarified] They can be hard to come by in the shops. Without being able to pick up and play with an Olympus in person, how can you choose it?
4. The e620 grip is small for some, though I think the problem here is exaggerated. Even when walking around town, the camera is around my neck so handling it by the grip alone isn't a factor. When taking pictures, I always support the camera underneath as a good habit so the grip on the e620 is again not a serious disadvantage.
I don't completely agree with the comment that most pictures sizes are 3:2 format. The ISO paper sizes are closer to 3:2 (they're actually [sq. rt. of 2]:1) in countries outside the USA. Here in the US at least, most of our print sizes are closer to 4:3 for medium size prints -- we start with 4x6 and 5x7 but then go to 8x10, 10x13, 11x14 and 16x20 before jumping again to 20x30 and 24x36. Annoying! I prefer the ISO sizes. Really, with digiral imaging, we should drop the 35mm legacy and all go with ISO proportions for sensors and prints. But that would be too convenient so it will never happen.
The Olys and e-series are not for everyone. I don't recommend them for wedding photographers or sports photographers. They're fine for indoor family stuff with the right equipment to deal with the low light, but others are probably better. They're almost ideal for hiking and landscape photography, especially with the weather-sealed lenses and cameras. They're good for street photography (assuming Leica is out of your reach for the street photography, a safe assumption for many) too. They're perfectly good for macro and portrait work though again other systems may have advantages over them. A lot of people do a little bit of everything when they take photographs, they aren't specialists. But they do *tend* to prefer certain kinds of shots or subjects. The Oly seemed excellent at what I do most though not exclusively, and good at most other things.