Feature:
I will be honest. There are other units around the same price range as the Duet that contain many more features, such as digital in/outs, MIDI capability, multiple headphone outs, etc. The Duet is rather light in the feature range. However, if you only require two simultaneous inputs and outputs, then the unit is perfect. MIDI gear typically interfaces via USB with many units, and MIDI interfaces are pretty affordable now. Digital in/out would have been nice, but the Duet's converters are wonderful, so I don't think I will miss that feature too much. Mac compatible only, but that's no problem for me.

Quality:
The quality of the Duet seems to be quite good from my initial impressions. Unit is sturdy, and the drivers are well made. Some have complained of the breakout cable breaking, but I don't think this will be a rampant issue with the unit (I would only think this would be a problem if you are traveling quite frequently with the unit). Anyways, there is currently a breakout box for the Duet available made by a small, third party company if you are concerned. The Duet itself is very well constructed.

Value:
If one thinks about it, many high-end two channel converters are anywhere from the same price to thousands of dollars (Mytek, Benchmark, other Apogee). While the Duet isn't necessarily in the same league, it still compares quite favorably with these more expensive units. Add in the fact that one can get two wonderfully clean, transparent preamps, plus a great headphone amp and both AD and DA, and the value of the unit becomes very apparent.

Desirability:
The unit is quite pleasing aesthetically. The Duet was designed very well, and it is a great match for my MacBook Pro (the finishes match up nicely). Features and sound quality aside, the Duet is very visually attractive.

Sound:
Apogee has been making high end AD/DA converters since the 1980's. They have built a good reputation for themselves with such units as the Rosetta 800/200, AD16/DA16, etc. That being said, the converters aren't the same as on their higher end units, but they do sound quite good. Out of many interfaces that I have used (PreSonus, Digidesign, and others), the Apogee would definitely get my pick for best under $500.00. Great clarity and depth, with good stereo imaging. The preamps in the unit are very clean and provide 75 db of gain. They aren't going to give you much character (like some higher end pre's), but they will give you a very clear signal from your microphones. I have used many different microphones with this unit, and have gotten good results from almost all of them. Even the SM7B, which is rather gain-hungry proved to be a pretty good match with the unit. DA is also very good. The only concern is that the outputs of the unit are unbalanced, so you may experience a slight hum (very slight), but nothing that would prevent you from accurately hearing everything. For the sound quality that you get from the unit, it is a worthy trade off.

Ease of Use:
Couldn't be any simpler. If you are familiar with any other unit in the same vein, you should have no problems at all. Interface is very, very intuitive and straightforward. I had the unit installed, running, and recording in under ten minutes. The Duet has a wonderfully simple, yet very capable control scheme. You can also push in the rotary encoder to select volume control for input one, input two, master output, or each of the four assignable MIDI controllers.

Support:
I haven't had to contact Apogee for service work yet, but they seem to be pretty easy to work with if one understands their policies and is a reasonable customer. Like every manufacturer, some have wonderful experiences, and some have awful experiences. Keep in mind, people who have negative experiences are more likely complain than people who have positive experiences that they can praise.

Overall:
Overall, I feel that the Duet is the best sounding interface of anything $500.00 and below, and beats many other units that cost more. However, this superb sound quality is balanced by a relative lack of features (as compared with other interfaces at the same price point). If you are only tracking one instrument at a time, and do not need MIDI capability in your interface, then I would definitely suggest taking a closer look at the Duet. For my application (recording only solo instruments), it's a perfect fit.