I recently purchased an E-P2 as a portable option to complement my DSLR setup. I've been very happy with its performance in most respects, however it has no flash and the kit lens is slow; this means that indoor usage results in painfully high ISO and noise that is impossible to clean. That lead me to purchase this lens, which is the fastest thing going for M4/3 currently.
First off, this thing is wonderfully compact. Much thinner than the M.Zuiko 14-42mm kit lens, although a bit bigger in diameter, and it makes the E-P2 even more low profile when it's mounted. It balances well on the E-P2 and it looks right at home to me, even though it lacks the signature PEN silver accents. It has a massive manual focus ring which is a pleasure to use (once you get the hang of the micromotor manual focus mechanism, which it shares with the 14-42).
Unlike the newer, smaller Panasonic Lumix 14mm f/2.5 pancake, this guy has some very large lens caps that make it more bulky than it should be. A lens this size begs to be slipped into a pocket, but the huge caps just get in the way; you might want to look for aftermarket replacements.
As for IQ... well, you can read the charts, but suffice it to say that this is a very good performer. Unlike the M.Zuiko kit lens, this guy is sharp wide open, with perhaps only minor improvements by stopping down. This lens can really show what M4/3 is capable of from an IQ perspective in a way that the kit zooms just can't.
Autofocus is at least as accurate as any other lens I've tried on this body, and low light AF accuracy especially is quite good (thanks, no doubt, to the large aperture). AF speed though is mediocre at best, and it's also a good bit louder than would be ideal. The aperture also makes a very noticeable clicking sound, which is a little obnoxious with the E-P2 (which constantly fiddles with the aperture when you're using live view). Neither focus noise nor aperture noise is a real issue for stills, but they might cause some problems if shooting video with the internal mic.
There is one issue with using this lens specific to Olympus bodies: they don't do the software distortion compensation that Panasonic bodies do with Panasonic lenses. This means that, on Olympus bodies only, light falloff is visible towards the corners when shooting wide open (it goes away by around f/4). In practice it's not much of a problem, and the corners aren't that big a deal in most shots anyway, but if you're a PEN shooter you should be aware of it and stop down accordingly if/when the corners do matter.
I should mention that I also own the Olympus 17mm f/2.8, which many would consider an alternative to this lens. If you're new to photography and you're considering owning only one, I highly recommend the Panasonic, whose aperture advantage and longer focal length combine to give you much better options for DOF control. I still enjoy the Olympus (and indeed, it's even *smaller* than this lens), but I think for most people in most situations this is a better choice.
Bottom line: if you own a m4/3 body without a flash and care at all about low light shooting, stop reading and buy this lens right now. For everybody else, you're presumably here for IQ, f/1.7's narrower DOF, and portability, and on all counts this lens delivers. It's a great all around performer and the cost, although high, is not totally unreasonable for something of this quality.
As a side note, if you're used to 50mm as "normal" from the film era, I think you'll find 20mm quite pleasant; I've shot a lot with my 28mm prime on APS-C, and the Panny 20mm here provides a similar FOV to that.
** Update, 5/21/2010 **
My initial thoughts on this lens have proven to be spot on. This little guy spends much, much more time on my camera than the kit lens; the faster aperture is a godsend, and the smaller size is wonderful for portability. Image quality is impeccable and the light falloff is almost never a problem in real world scenarios. If you only buy one lens for your M4/3 body, make it this one.
** Update, 8/7/2011 **
Panasonic has announced a µ4/3 native version of the Leica-branded Panasonic 25mm f/1.4, which should clock in at about twice the price of this lens. I own (and reviewed) the regular Four Thirds version of that lens, and early accounts imply that the µ4/3 version should be nearly the same optically as its full sized cousin.
The 25mm f/1.4 is certainly better, but not by much; only half a stop faster, barely any sharper, and still suffering from corner softness and vignetting wide open. The focal length is a more traditional normal, which (combined with the aperture advantage) makes it more suitable for isolating your subject, but... at twice the cost and twice the size? Those advantages seem difficult to justify.
As far as I'm concerned, the 20mm f/1.7 here remains the first lens you should buy for a µ4/3 body. It plays to the strengths of the system without breaking the bank.
** Update, 1/23/2013 **
I've finally taken the plunge and purchased both the Olympus OM-D E-M5 and the Panasonic Micro 4/3 25mm f/1.4 to go with it. I can now directly compare this lens to the native 25mm f/1.4.
I can honestly say the 20mm f/1.7 holds up incredibly well. The biggest advantage of the 25mm is probably its autofocus performance, which is both much quieter and somewhat faster than the 20mm (primarily when "hunting," which is when the 20mm slows down). The minor aperture advantage of the 25mm is really not very significant (especially with the E-M5, which has impressive high ISO performance).
Oddly, perhaps, I still end up preferring the 20mm simply due to how much smaller it is. The 25mm is superior, but it's relatively large, which means I'm just less inclined to carry it with me. So if you're considering these two lenses for yourself, you really must decide how much you value portability. The 25mm f/1.4 is "better," but the size difference is real.
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