Pros: nice, small size; understated beauty; inexpensive for a solid rosewood guitar
Cons: some variability across samples in playability, intonation, and sound quality
The Martin OM-21 is a "no frills" rosewood OM-style guitar from Martin. The OM style has the usual 25.4" scale length, but is a smaller guitar than Martin's famous dreadnoughts, like the D-28. The OM-21 also has a slightly wider fingerboard (1.75" at the nut), relative to Martin's standard dreadnoughts. OM-style guitars are said to be especially well-suited to fingerpicking, though most respond well to flatpicking as well.
I don't know if there are structural differences between the OM-21 and the OM-28, Martin's "regular" rosewood OM, but the OM-21 is a much simpler guitar cosmetically. In fact, its looks remind me of a smaller D-16 or D-18, two mahogany Martin guitars that come with relatively understated ornamentation. A quick look at the 21's specs on Martin's website confirms that some of the 21's appointments are in the "18" style, while others are in the "28" style.
The OM-21 is a solid rosewood guitar with a solid spruce top. The fingerboard of the OM-21 is rosewood, not the ebony of most of Martin's snazzier guitars, including the HD-28 or OM-28. The neck is made of mahogany. Top bracing is scalloped 1/4", which should lead to good responsiveness and projection. There are 14 frets clear of the body on this model, and no cutaway or electronics, though electronics are available as an option. The retail price is in the $2000 range, But you should be able to get a new one for about $1500.
Because of its small size, the OM-21 is very comfortable to hold and play. As with my Santa Cruz OM, the lower bout is small enough that it doesn't lead to shoulder pain in the right shoulder during long playing sessions.
Playability and intonation have varied somewhat on the samples I've played. Some have had moderately high action and only good intonation. Others have had medium-low action and really good intonation. I prefer the ebony fingerboard of the OM-28 to the rosewood of the OM-21, but the fingerboards on the OM-21s I've played have been of high quality. Some players actually prefer the slightly greater resistance of a rosewood fingerboard.
Tone quality has also varied on the samples I've played. Some sounded a bit dead, lacking the responsiveness I'm used to with scalloped-braced guitars. Others have been nice and lively.
On a good OM-21, the sound has a nice balance between rosewood fullness the delicacy one expects from a smaller guitar. The sound of the OM-21 is a bit thicker than that of most comparably sized mahogany guitars, but the bass isn't as overwhelming as on a dreadnought, like the Martin HD-28.
The OM-21 does have the woody resonance that tells you it's a Martin, not a Taylor or Larrivee. Its sound is not as bright as that of most Taylors, nor as liquid as most Larrivees. But there are days when I need That Martin Sound.
A friend of mine is an excellent fingerpicker who plays in drop-D tuning a lot. I've mentioned him in other guitar reviews. The Taylor 612CE, which sounded great for my style, sounded a bit too lightweight in his hands. But the OM-21 suits his style perfectly, the bass perfectly balancing the trebles when he's playing Irish tunes. He loves this guitar. In fact, he had a used one in his sights down at Gryphon Music near Stanford University, and regrets to this day that he didn't snap it up. The price (used) was only $950.
In conclusion, if you are a fingerpicker whose style is suited by a woodier, slightly fuller sound (as compared to the more delicate and lightweight sound of Taylors and Larrivees), and you're a bargain hunter, give the OM-21 a look. But be sure to play several samples. If you prefer an ebony fingerboard and a bit more ornamentation, check out the Martin OM-28, OM-28V or Santa Cruz OM. I've heard that Larrivee's and Collings's OMs are also excellent, but I haven't had a chance to play them yet.