One of the perks of this job is that I get to play lots of Gypsy guitars...all kinds of instruments from the most inexpensive Asian copies to vintage Selmers. In doing so, I've learned a lot about the wide range of tone and playability that exists in these instruments. Ultimately, "good" is a subjective thing, since any given player may love a guitar that I hate or vice versa. But in general, I think there are some basic rules you can use when looking for a Gypsy guitar. The following are my thoughts and experiences:
Four characteristics a Gypsy guitar must have:
1) Bright, trebly tone
This is a hard one for people coming from the flatop, classical, or archtop world. Most people are accustomed to looking for a balanced, full sounding guitar with a very present bass response. But really what makes a Gypsy guitar useful is it's ability to spit out lead lines loud and clear. This usually comes at the expense of balance and fullness of tone. But in the right hands, these guitars sound great. Basically, a Gypsy guitar is really a mandolin with six strings.
One of the biggest mistakes newbies make is that they buy a guitar that looks like a Selmer but sounds like a flatop. They still have their old sense of aesthetics, and avoid the trebly sound of most Gypsy guitars. Most eventually figure this out and trade to something with more punch.
2) Relatively Dry Sound
When you first play a "wet" or reverby sounding guitar, it can sound really amazing. The reverby quality of "wet" guitars gives them a big, full sound with a lot of character. However, what sounds good alone, doesn't sound so great in a jam setting. Once you start playing with someone else those reverby qualities work against you. They make the guitar sound small and nasal. Most of the best Selmers, Favinos, and Duponts I've played were very dry, or maybe slightly "wet," but never that reverby sounding. A dry sounding guitar really cuts and sounds full when playing in a group. But can sound a little dull alone...so beware!
3) Lack of annoying overtones
Some Gypsy guitars will give you a whole orchestra's worth of overtones every time you hit a note. Again, this is something that sounds kind of cool when you play the guitar by itself. But is ultimately counterproductive and annoying when playing in a group. I like to hear a really strong fundamental when I hit a note....it drives me nuts if there's a bunch of overtones and sympathetic strings ringing. Sometimes people ask me about palm muting...my guess is that these people have a guitar that produces these sorts of overtones.
4) A hollow bass sound
As mentioned before, you really don't want a big fat low end on a Gypsy guitar. It takes away from the guitars main job, which is producing crystal clear lead lines. But Gypsy guitars can have some bottom. The really good Gypsy guitars have what I call a "hollow" bass sound. It's almost like you're hearing a subharmonic without much of the fundamental. Not sure, but whatever a spectral analysis might reveal, it sounds sort of "hollow" to me. But in a good way. It gives the feel of a bass note without really taking up too much sonic space.
Anyway, those are my thoughts. I'd like to hear what other people think. I only discussed tonal characteristics here. At some point we should discuss playability as well.