I have been using an unmatched pair of sm81s since 2008. Although unmatched, the mics sound very close to each other. The mics are rugged and built to last.

The sm81 has a very flat frequency response and this makes the mic useful for a lot of acoustic instruments. It’s sound is more rounded and less crisp than my Peluso cemc6s, which have a rounded peak at 7khz. The sm81 is a little punchier than the cemc6 though, in the mid-range. I tend to use the sm81 in bright rooms (e.g., no carpet) on brighter instruments (e.g., Taylor guitar) and use the cemc6 in darker-sounding rooms on duller instruments (e.g., Martin guitar with dead strings). If I have a guitar player with no rug under him, I pull out the sm81s, or add a rug and use the cemc6s. Room untreated? Use the sm81s for sure. Thus, the sm81s tend to be a little more versatile than my cemc6s.

I used to use the rapid bass rolloff setting for acoustic guitars, and after awhile I came back to the flat or gradual setting, as the rapid rolloff takes out a lot of low end. The rapid rolloff is best reserved for use to remove a problem LF hum. The gradual rolloff is the more preferred setting for acoustic guitars if you are getting too much boominess. Overall though, I use the flat setting (no rolloff) most of the time without problems. Using the pair in stereo configuration imparts a very big sound on acoustic guitar.

I have successfully used the sm81s on choral ensembles, piano, hand percussion, and all manner of stringed instruments, including fiddles. I have seen Alison Krauss use the sm81 onstage for her fiddle. I have used the sm81 for male vocals as well, where it did surprisingly well, albeit a little wooly-sounding if you get too close. I definitely need a pop filter to use this mic for vocals, but I have heard of barbershop quartets using sm81s in live performances.

With a 5.6mV/Pa sensitivity, the sm81 takes a bit more gain than other SDCs. It sounds great through a Sytek or Grace m101 pre.

At one time this mic might have been a deal in its class.
However I feel there is competition now that squeezes this design out a bit.

Maybe its just me but I find the high end a bit tinny on this mic. Not that its not a good usable tool, but I cant recommend it when shures newer offerings seem much improved, and lineaudios CM3 to me is the king of inexpensive sdc's topping them all and competing with mics 10x more expensive.

Now if you find one of these used at a good price well then you can definitely get good results.I think there best application is acoustic guitar. Depending on the source.of course.

While its a good workhorse I feel it is a mic whose time is up , there are too many options close in price that I prefer for their tone.
Shures new sdc's
Oktava mod the rode mics and especially the Cm3 all are in the same category id recommend comparing these to an sm81 and see for yourself what you prefer it may be the sm81 but I dont.

I first heard these mics doing a stereo recording of a string ensemble. Let me tell you I had to get some! Since I've used them for strings, guitars, mandolins, tambourines, cymbals, hats, OHs, etc. They're pretty much awesome. I got a used pair on ebay for $350, so there's deals out there everywhere.

Need a stereo set up for field recording, these are your guys.

Rock solid, can take a beating, -10db pad and two hi-pass settings. Great response across the entire spectrum. I love these mics!

The SM-81 is a nice Small diaphragm condenser, which used to be mfg. by Shure in the USA, has been around since 1978, and sounds like it. That can be a good or a bad thing depending on your personal preferences. The sound of the mic could almost be called "vintage". It has an accurate but slightly colored sound, more easily apparent when A/B'ing the mic against newer, "faster" SDC's.
But at the same time, that color can be part of the charm of this mic. Oft used in recent years in recording studios and on stages around the world, the sound is malleable and usable on many sources. Not strident or overbearing in any area of the spectrum, it is a real workhorse that can always find use on any session or live gig.
Worth having one or two, especially if they are found at a discount, the SM 81 has worked its way into the history books as a standard, and a mid-level classic.

I'll start with bang for the buck... I have one I bought used for $100.00 ..I would buy another for that price. or maybe a little more...
the price of a new one is a bit high when looking at all the competitors.

features .. the built in pad and roll off are nice .. it is not self positioning like the .....hmmm none are.

Ease of use and sound quality..

I don't consider this a 'pretty' sounding mic.. its flat and fast.. if I want to get more percussive sound out of an acoustic, more ping on a cymbal, more stick on a drum, this is the choice ... on the other end of the spectrum (of the mics I have) is the Oktava 012 it is slow and not so flat ...but kinda pretty sounding.. If I had to choose between the two I would rather work with the SM81 as i find it possible to slow it down with a little compressor and EQ. and maybe a foam pop thingie on the end....as a note I find it impossible to speed up the Oktava.

I also have a KM184 which is the most used of my 3 SDC's.. its quite pretty sounding .. fast enough and flat enough .. but it does not have the presence and punch of the SM81 ..

the SM81 is not a 'forgiving' mic

if I was recording a really hot sounding acoustic guitarist like Michael Hedges (rip) or a really up front fiddle or dobro (played by someone who needs no forgiveness from a mic) I would definately try the SM81 first..

I guess 'agressive' is the word I would use if I had to use only one word.

best

I have a a pair of early USA made SM81's. Love them on hi hats as it seems to add a little bit of warmth to the sound, and somehow rolls off the sibilants. I've used it as a side of snare mic and it worked really well there too. Haven't tried them as OH's yet, but that's next on the docket.

I've got a pair of SM-81's. The features are pretty well known, electret condenser, cardioid pattern, flat response, 10db pad, and a selector switch with three settings for low end roll off, flat, 6db below 100hz and 18db below 80hz. There also used to be an omni capsule available for it, SM-80 I believe. Shure is great for build quality and the SM-81 is no different. They do not come in proper matched pairs from the factory but there seems to be a pretty reliable consistency so it’s not that much of an issue. I will add though that I have occasionally seen some older SM-81’s that were 2-3 db lower in output than newer models so if buying used check them out.

The SM-81 is great for drum overheads, acoustic instruments etc. It's fast and detailed and doesn't hide much, what you put in is pretty much what comes out. Plenty of presence but not overly bright. The roll off settings are useful when you need them. It has pretty decent off axis rejection also. With a little creative mic placement I’ve been able to get usable takes with minimal bleed using the SM-81 on guitar and an SM-7b on vocals concurrently. The SM-81 does seem to want a little more gain on softer sources compared to other SDC's I've used. On acoustic guitar I'll use a spaced pair or a single sitting over the 12th - 14th fret area, sometimes with a ribbon mic a little farther out to add a sense of space with great results and usually requiring very minimal, if any at all, any corrective EQ in the mix.

If you can't get it done with an SM-81 the problem is not likely the mic.

I have my 81s since about 25 years and still love them.
I use them for my grand piano at home and for my Leslie treble horn on stage. Additionally I use them for recording classical ensembles like chamber music ensembles or choires - along with my Neumann TLM103s.
Someone stated that they are not as neutral as modern mics - more kinda vintage. I don´t know what "vintage" means. To me they sound pretty natural. But I start from the premise that even the most expensive mics don´t paint a true picture of a natural sound. There are always certain differences - to the better or the worse. So in the end it´s always a question of taste and experience of course.
Besides - I think that the SM81 offers a lot for it´s price: great sound and versatility. So I´m just about to buy another pair.