Quite Awesome for Acoustic Guitar
I have recorded 'ukulele, acoustic guitar, and upright bass on this mic. And I am very much pleased with the natural sound that can be obtained from this skinny little microphone. It's definitely a great mic to have in your locker for all you acoustic musicians and engineers.
For 'ukulele, I recorded with just the SM81 about 8-10 inches away, in front of the 10th fret. The 'ukulele I used is not my favorite 'ukulele, it's a spruce top with maple back & sides, so the sound really isn't too great. But the SM81 did translate it pretty well. It sounded just like the 'ukulele.
When I recorded acoustic guitar, I used a Takamine dreadnought, spruce top and mahogany back & sides, which sounds very, very nice, with an even tone. I had the SM81 about 12 inches away, directly in front of 12th fret. It did a really good job with translating the sound. The only thing I would say is that as I listened back to it, it did catch a bit of boominess, but it was easily taken out by some simple eq. I cut a little of low-mids and mids, and boosted a little of high-mids and highs, and now, the guitar sounds natural and sits very well in the mix.
When I recorded upright bass, I used this mic in combination with a LDC, about 4 or 5 inches away from the body and it worked out very well. It caught the true tone, nice and even, very little eq-ing and it was ready to go.
Like I mentioned, it is definitely a great mic to keep in your locker. It is also a good mic for recording choirs. I also know a guy who used to put these on his ride cymbals to catch a little more sizzle and I did end up liking the sound of them pretty well. If you record a lot of acoustic stuff, don't hesitate to get this mic, it's very great to have around. Buy a couple maybe, just to have that stereo pair option.