Nextel has the advantage of TCP/IP stack already built in (Public MDS/MDG). Already works with Internet applications such as instant messaging, chat software, telnet/SSH.
Also permits static IP addresses to be assigned directly to the devices for an additional approx $4 per month. Nextel is one of the few networks with native TCP/IP at the wireless network level without NAT/gateways being necessary. You can't get a static IP address from any other carrier without getting BES/MDS -- celltower changes and coverage going in-and-out can change IP addresses and disrupt instant messaging and telnet/SSH sessions.
Coverage is good in some areas. But in other areas, you may prefer another carrier such as CDMA or GSM. And you pretty much can't roam outside of North America if you stick to the iDEN network.