Concept: 3 out of 5
Execution: 4 out of 5
Yeah, but: There's no reason for a 4/3 shooter to not own one.
The Long Version: 50mm lenses are something of a cult classic, and it's a really big cult. There's a mystique about the Leica-equipped starving artist photographer - starving because they spent $5000 for a light-tight box - stalking the streets with a fast fifty lens, looking for decisive moments and refusing to crop their photos even though their viewfinders are woefully inaccurate. In this era of cheap slow zooms, the 'nifty fifty' lenses can be an affordable way to get better image quality without sticker shock. (True story: when I tell people the price difference between the Nikon 55-200 and 55-200/VR, I often get a stunned look and a feeble "...oh, I'm not a pro" response.) There's also an argument, typically oversimplified and overemphasized, that a 50mm lens gives 'normal' perspective and most accurately reproduces how we see the world.
Fortunately, the Olympus 50mm is completely above all that. In the Olympus world its focal length makes it a short telephoto, and it lands perfectly in the most poplar macro and portrait focal lengths for legacy film cameras. It's also not cheap, although its price is aligning nicely with the new generation of 50mm lenses, and it's far less expensive than macro lenses with similar fields of view for other formats.
The 50/2 is one of the smaller Olympus lenses, making it easy to carry in any bag and easy to use on any camera. The build quality is a very solid plastic with a metal inner barrel. When shooting at close distances it extends enough to make some ladies faint, but at anything farther than a few feet it's perfectly suitable for use in mixed company. It's fully sealed against dust and rain, and the barrel-shaped hood is deep enough to keep water off the glass. This makes it my second choice for shooting in bad conditions, and since the 35-100 is my first choice, there's no shame in that.
The 50/2 macro comes in second place a lot. I've been told that it's the second-sharpest lens from Olympus, with the 150/2 taking first place. It's usually grouped in with the 35-100/2 and the 150/2 lenses as the best choice for portraits. That may not sound like a great endorsement, but those other lenses cost five times more than the little 50 Macro - each. I actually bought the 50 Macro specifically for low-light portraits, and have used it as a general-purpose lens far more than for macro photography.
In addition to being much more affordable, the 50/2 is petite compared to the Top Pro monsters. It's a much easier lens to handle and not nearly as intimidating, which will often make it the best choice for portrait and candid photography. Its biggest limitation here is the absence of a focus limiting switch, so occasionally it will want to check to see if there are any interesting dust motes nearby. The better autofocus systems in the E-3 and newer cameras cuts down on this, but it's still not a great lens for fast action.
Shallow depth of field isn't the strength of the four-thirds format, but the 50/2 is one of the better lenses for it. I've never hesitated to use it wide open at any focusing distance. SLRGear and DPreview have both done their own version of stringent testing on the optics, so I won't go into further detail here than to agree that this lens is nearly perfect.
The Olympus 50mm really needs to be considered alongside the 35mm Macro, since the difference between them doesn't seem that great. The 35mm was the second lens I bought, and it served me well for a long time. Like the 50/2, it's an exceptional lens: small and sharp, and less expensive than its bigger brother. But lenses cost more because they really are better. Once I had the 50/2, its faster aperture, weather sealing, and (relatively) better autofocus, combined with a focal length that I preferred, meant that the 35/3.5 only got used when I really needed 1:1 reproduction. The Sigma 150/2.8 macro (reviewed) also does 1:1 reproduction (the 50/2 is half-sized, 1:2) but with a much better working distance, so the 35mm Macro found itself a new home where it continues to provide exceptional value for the price. Compared to the Sigma 150/2.8, the Oly 50/2 isn't as good as a dedicated macro lens, but it spanks the Sigma for everything else.
The 50mm Macro lens isn't the best for infrared photography. At narrower apertures it shows a distinct 'hot spot' in the middle of the frame. It's evident at f/8 and above, and I can just make it out at f/5.6, but I don't see it at f/4 or below. Sometimes it isn't an issue in real-world photography - the image above was shot at f/8 - but typically it is. IR requires long exposures, long exposures need tripods, and tripods encourage stopping down. This is particularly unfortunate because the 52mm filter thread makes an IR filter, like the Hoya R72 that I use, much more affordable. The 35mm gives better results, but the cheap 14-42 is (was) my favourite for IR on an Olympus SLR.
Miss Piggy provides me with my favourite photographic advice: "Never eat more than you can lift." The 50/2 is so easy to carry, and so unassuming to shoot with, that it would be worth the price even if it wasn't such an excellent and versatile performer. In many ways it's a very boring lens to use (and review), simply because it does everything so well and consistently that it becomes unremarkable. It is transparent in ways that go far beyond its remarkable clarity; if it wasn't for the occasional hunting and reassuring buzz of the focusing motor, you'd forget it was on the camera at all.
If the children are fed, the rent's paid, and you have an Olympus or Panasonic dSLR, you should own the 50mm f/2.0 Macro. It really is that simple.