Manufacturers usually state 100 to 300 feet range depending upon the environment. By adding higher gain external antennas and or amplifiers the range can be greatly extended. The limitations are the antennas, LOS, and the curvature of the earth. To put that into perspective, wired 10BaseT ethernet is a full 10 Mbps, and a 56K modem connection is .056 Mbps. Wireless Ethernet is insecure by default.
Wired connections are generally more secure when communicating with other servers.
All devices on the market are approved by the FCC, which has a good public document on RF Safety . Since this is a group of independent volunteers, compatibility may vary between access points. It is possible to use other operating systems on NYCwireless. Any 802.11b compliant card should work. People have also had success in using Prism based cards.