Even though I received the GW-3 from B&H at the end of last year, it took me a little while to test. Finally, since I had some vacation time planned at the end of January, I was able to put the 21mm lens through its paces in Mexico. But before I comment on the glass itself it probably makes sense to talk about the set-up and handling.
First off, set-up: To affix the 21mm converter you need to separately buy the GH-3 lens adaptor and hood. While it's easy enough to screw on, there are at least two things that are a little off-putting about this. One, both the hood and adaptor have the look and feel of cheap plastic (which they are) and do not really suit the build and feel of the GR. Two, it is galling that the GW-3 can be affixed to the Sigma DP cameras directly without the GH-3 while the dedicated Ricoh user is expected to buy an additional piece of equipment to do the same.
Second, look and feel: The actual lens is a beauty with a solid feel that adds weight and character to the otherwise pocketable GR. The rubber lens hood is another matter altogether, fiddly and vulgar. In fact, given that one cannot really use the GW-3 without the GH-3, one wonders why the rubber lens hood was included at all. The GH-3's hood, although matte finished plastic, would have sufficed.
Lastly, the handling: You have to remember to switch on the conversion lens option from the menu! To ensure I did not forget this, I assigned all my personal settings to MY3 along with the conversion lens setting to "on". This way, I could toggle between MY1 and MY3 every time I unscrewed or screwed on the conversion lens. The other suggestion I have is to use a monopod or, even better, a tripod while using the lens. This is because (and I am working on the assumption that you have used the Ricoh GR long enough to believe that it feels just right in your hand) the extra lens sometimes make the overall setup seem too heavy when shooting, especially if you are used to shooting with one hand.
Once you've reconciled yourself to all of the above, there only remains the proof of the pudding ... image quality. Below are some samples from my (rather brief) sojourn to the dispossessed Mayan walled city-by-the-sea: Tulum.
The Ricoh GW-3 is a very competent performer and, given its relatively low price (you can even pick one up brand new for ~$200), it is worth the expense. Chromatic aberration was missing in the GR, and is virtually absent after the addition of the GW-3 lens. There is very slight barrel distortion (and nothing that cannot be fixed in PP). While the 28mm lens id sharp corner to corner from say F4 onwards, to get the same level of sharpness from the GW-3 step down to F7.1 or F8. Else (F2.8 - F5.6), the lens is sharp in the centre and at the corners, with image deterioration restricted to the corners. If you are fond of vignetting, this problem will be taken care of automatically.
Ultimately, I was impressed by the GW-3. I can see myself using the lens for general landscape photography, street photography (where I want more context) and architecture (skylines, etc.). Where it may suffer, and I have not had a chance to test this yet, is landscape-in-low-light (where a larger aperture is required) and interiors (where detail is needed corner-to-corner). Despite this, the GW-3's compactness, sharpness and versatility make it a keeper.