Excellent on Guitar, but works well with other instruments
When I purchased the DPA 4099G, I also purchased the DPA4099 Piano pair too. My initial review can be found on the 4099 P page, as I gave a general review of all of them at the same time.
That being said, these mics are really smart investments, and for anyone who runs live sound or records live shows, I'd encourage you to buy several of these rather than investing in one boutique microphone. I record live shows in a theater space in the upper west side of New York City, and the range of shows runs the full gambit from classical to jazz to rock to world music. The challenge, since we're a small theater, is in isolating instruments in the mix. The 4099s have helped tremendously with this. I won't say that they completely isolate the instruments from others in near proximity, but they make the post performance mix down much easier. The design is brilliant, and the time spent getting the desired sound is minimal. So, for live sound folks, it's a quick set up, and even shorter amount of time in finding a sweet spot. Guitarists (and other held acoustic instruments) have a little more freedom of movement too. I also recommend that if you can only afford the guitar mic, consider buying a few of the other clips. That way you can interchange the mic with other acoustic instruments.
If you can afford more than one, DO IT! You won't be sorry. The smiles from the players at how quickly they like what they are hearing is a real perk.