I needed this primarily for recording personal coaching sessions both in person and on the phone. I wanted a stereo recorder that saved files in a non-proprietary format such as WMA or MP3, and I needed it to run on an external AC power supply as well as on batteries. This is one of very few recorders that meets those requirements.
At first I was disappointed because, out of the box, you cannot make recordings while it's plugged in to the power supply. The default configuration assumes that when the USB connector is plugged in to the power supply, you are really doing file transfer rather than recording. In this respect, my old Olympus DS-40 is better, because the external power supply connects to a mini power jack that is separate from the USB port. Another non-intuitive difficulty I encountered in using the power supply is that it will NOT recharge the internal batteries unless you hold the "STOP" button down while plugging in the power supply.
It takes some digging through the PDF manual hidden in one of the recorders file folders to discover the "USB Connect" configuration settings, where I was able to change the configuration to ask on startup whether I was charging the device in PC mode or in recorder mode. The quick start printed manual does not contain that information. Frankly, I prefer the intuitive simplicity of the separate power connector on the old DS-40. Also, the DM-420 manual says you cannot leave regular alkaline batteries in place while running the recorder on the power supply, and yet the power supply does not work if the batteries are removed. My old DS-40 can run on the power supply with batteries removed, and it doesn't care if the batteries are regular or rechargeable because it's not trying to recharge them. So, having the capability of recharging batteries inside the DM-420 is a mixed blessing. Because I use the recorder in my office always connected to the power supply, I've almost eliminated my contribution of dead batteries to the environment.
Over all, the operation of the external power supply in simply recharging batteries, or making recordings while on the power supply, or operating as a replacement for completely dead or missing batteries, or recharging the batteries while connected to the PC via the USB cable and transferring recordings, is very non-intuitive and requires careful reading of the manual. It shouldn't be this difficult, because my HTC smart phone, which I also use as a recording and playback device, is completely transparent to use of the external power supply/charger which also connects via the USB port.
My use of the recorder is not completely restricted to the office. I carry it to the gym, on walks and on the airplane in order to listen to music and books on tape. With the microSD card slot, I can carry hundreds of hours of audio.
The quality of audio from WMA and MP3 music files that I copied to the device are excellent, and the stereo recordings I've made in the office are excellent (using two separate left/right microphones connected via a stereo-to-mono Y splitter). Recordings made from the phone are monophonic, but also quite clear, being limited by the phone line quality not the recorder.
I strongly advise any purchasers of this device to at least skim through the detailed (128 page) PDF manual that's hidden in a folder on the recorder itself. The recorder can be highly configured to a wide variety of different recording and playback possibilities, and if you don't read the manual, you will definitely be missing out on a lot of possibilities. I copied the PDF file to my PC and printed the whole thing out using the duplex and 2-up paper saver settings on my printer - it was well worth the effort, and only cost 32 sheets of paper.
By the way, the external AC power supply/charger (Olympus A514) is NOT included with the DM-420 recorder being reviewed here. You must order it separately. Otherwise, you are limited to recharging the batteries of the DM-420 by plugging it into the USB port of a computer.
In conclusion, I think this is a great little recording and playback device, in spite of the faults found in its user interface. The complex user interface conceals a great degree of flexible configurability, accessible only by careful reading of both the quick-start and hidden manuals. If you are one of those folks who never read the owner's manual, I'd hesitate to recommend this device to you.
Amendment:
Some folks have complained that when they use drag and drop to copy music to the recorder, they cannot find and play it on the recorder. If you have a microSD memory card in the recorder, you may have copied music to the microSD card but the recorder is still looking only at the built-in memory. I made a lengthy comment on Dr. Jerry Stettheimer's review in which I describe the procedure for switching your recorder from the built-in memory to the microSD memory.
I've been using the recorder for 12 1/2 months now, no problems. I must say again - reading the manual is VERY important to get the most out of this device. The BIG, DETAILED COMPREHENSIVE manual (128 pages) is concealed as a PDF in a folder in the built-in memory named OL_MANUAL. You will need to copy that into your computer and use Adobe to open and read it. The printed "quick start" introduces you to about 10% of the capabilities of this recorder. If all you read is the quick start brochure, you might as well have bought an old cassette recorder.