I'm not test D-Link modem, but I'm works with similar Lucent and AiroNET modems. The maximum real speed of 54 Mbit modems on our measurements is about 36 Mbit/sec.
NOTE: do not try to use this modems in poit-to-point connections with amplifiers on a long distance. It's protocols not works, when the time delay between points exceeds one.
Yes, it is useful. The distance, when connection breaks (with 30 dB amplifiers !) is about 3 km. When the distance is about 1 km, the connection speed slumps.