Should DES be avoided when developing new systems?
Yes. DES is no longer considered an approved algorithm by NIST, and breaking a 56-bit key can be done quickly - in 1998 the EFF DES Cracker managed to break a DES key in 56 hours. The average time it required to break any DES key was 4½ days. You could use triple DES instead, which is still considered secure, but it's very slow (in software, even single DES is slower than AES), and a time-memory tradeoff described by Martin Hellman in 1980 shows how to break 3DES with a 168 bit key in $2^{112}$ time. In addition the 64 bit block size is a major practical problem if using 3DES in some modes, as described by Thomas in his answer.