I owned a Duet for years and it definitely had some good points. I was playing Gypsy Kings, Strunz and Farah and some Brazilian music.
The Good;
The duet really helps with body taps since it has a built-in mic. It also helped with mellowing the quack of the piezo. Of course you can get too much mic and have feedback problems, but I found that it handled higher volumes better than full acoustic guitars. The smaller body and access up the neck were also very nice for gigging. I found it to be very versatile.
The Bad;
As with most mass produced instruments there can be a huge differences in each instrument. My friends and I owned 4 duets and there were huge differences in acoustic properties, pup response and intonation
. The cedar top with a satin finish typically gave the instrument a more dynamic and open sound even with the lack of body. The high gloss spruce held up better to body taps but sounded fairly dead (when directly compared to the cedar/satin). I didn't like the 1 7/8" nut, the radius of the fret board or the fact that the SA version was the only one that came with an ebony board (but these are personal preferences).
Overall;
If I was still gigging.....I would probably buy a Grand Concert Duet and be quite happy, but would still try and stick a mic in front an actual acoustic whenever I got the chance
.