I'm not really shocked that it works without an internet connection, especially with captive network login being a feature that's advertised as coming soon. Even though the interface is extremely customized, the Fire Stick has more true Android at its heart than the Chromecast does.
Reports are XBMC/Kodi is working well on the Stick after side-loading... I plan on giving it a go this weekend. I keep expecting SPMC to make it into the Fire TV section of the AppStore to eliminate the need to side-load, but so far it has not. Once I put XBMC/Kodi on, I'll probably go as far as setting it up to launch on startup, setting the home-button to return to XBMC, and launching other apps (Netflix, Hulu, etc) from within XBMC masking the Fire TV interface completely.