ok, I was reading about it late last night, so I might have missed the pre-assembled ones. Most of the adapters looked like this one:
where you had to Solder the board ($10), buy the XBee controller ($25) and the special FDTI Cable ($20). That's what triggered me to look at the other option, since it was cheaper and "all in one" without special cables and such.
Right, they say "The kit includes a PCB and all components necessary to build the adapter, unassembled. Tools are not included. This kit is really easy and will only take 15 minutes to build, even if you've never soldered before. " Assuming it's really 15 minutes it's not a big deal given the level of flexibility it gives.
However USB stick might be better idea interoperability wise if the goal is to work specifically on Zigbee, which is just a subset of IEEE 802.15.4
UPDATE: just foundCompatibility Note:
XBee and XBee-PRO 802.15.4 modules are compatible with other devices that use XBee 802.15.4 technology. These include ConnectPort X gateways, XBee and XBee-PRO adapters, and other products designated with the 802.15.4 product name. Devices that do not have the 802.15.4 product name, including Digi's line of DigiMesh and XBee ZB products, are not compatible with XBee and XBee-PRO 802.15.4 modules.