I can’t tell you what radio professionals in general are using, but as a professional print journalist, I have been happily using Sony mini-disc (MD) recorders for several years. And the first time I saw one being used, it was by a radio reporter.
Mini-disc recorders are small, convenient, relatively inexpensive, and provide excellent audio quality. As with most digital formats, you can choose several different modes, trading quality for recording length. Discs are relatively cheap. Standard discs are available for less that $2 each, and the high-capacity 1GB discs are about $6. Standard discs can hold between 2 and 10 hours of stereo digital audio, and the 1GB can hold between 8 and 32 hours.
The newest generation of MD recorder, known as Hi-MD, allows you to copy the recordings directly to your computer and work with them in digital format, although you’ll have to put up with some fairly crappy, DRM-laden Sony software. But the original MD format could only be copied to your computer through the analog headphone output, in real time, so Hi-MD is an improvement. Hi-MD devices can connect via USB, and be a storage device on your computer for any kind of file.
The advantage of MD over solid state digital recorders is that you can fill up as many discs as you want in the field, without having to dump the recordings to a computer once the memory is filled up. If you want, you can keep the discs as a permanent record, although you can, of course, erase and reuse them.
As others have said, the key to a qualty recording is a high-quality mike. My first MD recorder cost me $125 on eBay, but I paid $150 for the mike. Research carefully and choose wisely. (I bought this one.)
I use my MD to record interviews and speeches at conferences I cover for my publication. I used to use micro-cassettes, and those recordings were often unintelligible when I tried to transcribe them later on. But even at the lowest quality (which is all I need) the MD will pick up someone whispering on the other side of a large room. In stereo.
Here are several sites with more info on MD:
Minidisco, which has a helpful FAQ as well as other good info about MD and a reasonable selection of hardware.
Sound Professionals, a good source for good mikes.
Mini-discussion, a forum all about MD. Go here for info about how to overcome some of the shortcomings of Sony’s software.
Feel free to ask any questions you may have. Good luck.