We evaluate the performance of an IEEE 802.15.4a ultra-wide band (UWB) physical layer, with an energy-detection receiver, in the presence of multi-user interference (MUI). A complete packet based system is considered. We take into account packet detection and timing acquisition, the estimation of the power delay profile of the channel, and the recovery of the encoded payload. Energy detectors are known to have a low implementation complexity and to allow for avoiding the complex channel estimation needed by a Rake receiver. However, our results show that MUI severely degrades the performance of the energy detection receiver, even at low traffic rate. We demonstrate that using an IEEE 802.15.4a compliant energy detection receiver significantly diminishes one of the most appealing benefits of UWB, namely its robustness to MUI and thus the possibility to allow parallel transmissions. We further find that timing acquisition and data decoding both equally suffer from MUI.