There shouldn't be a reliability problem with USB Wireless Adapters, although PCI or PCMCIA would be the preferred option. USB was not designed as a "networking" technology.
Also bear in mind that USB 1.1 only supports a data transfer speed of 12Mbps which will restrict an 802.11g adapter to that speed. USB 2.0 would let it perform at its correct speed of up to 54Mbps. This is unlikely to be an issue for broadband internet access, but is significant for the transfer of large or numerous files between computers in the network.