If you're looking at buying a used DAT machine for recording, please consider this disclaimer: fewer and fewer companies are servicing DAT machines, as replacement parts are becoming scarce.
Also, finding blank DAT tape is becoming much more difficult as more companies stop producing blank media. Your best bet for field recording is now either hard disk recording, or Flash/SD memory recorders. DAT, compared to current technologies, is outdated and thus expensive to maintain and use, even though the initial investment of equipment will be quite small.
What is DAT?DAT is quite simply music stored digitally on 4mm magnetic tape. DAT tape has generally come in lengths around 60 minutes in length. However, most tapers go back and forth between using DDS-4, data grade tapes in lengths of 60 meters (2 hours) or 90 meters (3 hours). Some tapers have used 120 meter tape, which gives you more time; however, this practice is frowned upon because the tape itself is quite a bit thinner. This maximizes recording time, but unfortunately, some DAT recorders and players can't effectively handle the data-grade tape due to it's thinness.
DAT is great for music recording because it's bit-perfect when digitally copying a digital source. This made it a favorite mixdown medium for recording studios, since you could make a perfect 16-bit, 48Khz digital copy of your final mix, capturing all the nuances of a good analog system. Also, small portable recorders such as the Sony D8 and Tascam DA-P1 made this a perfect choice for tapers.
The Downside of DATDAT is a great medium, but quite simply, hard disk recording is more reliable, cheaper per hour, and the equipment is much less expensive to maintain. DAT also requires real-time conversion to move from tape to hard disk. Recording directly to hard disk negates this, and allows the user to have a finished product much faster. You're also limited in audio specs; DAT is only able to record 16 bit, up to 48Khz sampling rate.
DAT equipment is no longer in production by many major manufacturers - Sony stopped production of their last model in December, 2005 - and a lot of retailers aren't offering DAT products any longer. Due to the fact that DAT never caught on with a wide-scale consumer audience, there isn't a large base of repair centers that can, for an affordable price, fix DAT equipment. This has not only forced the price of DAT equipment down to new lows - a Sony D8 can be had in good condition for around $250 - but has made it harder to repair that equipment when it goes bad. Some places such as Pro Digital, a company that specializes in DAT, still offer top-quality repair service.