What interfacing do they have that the PICAXE community would find compelling?
Their best use may be in being smart peripherals for a PICAXE, delivering things which the PICAXE doesn't support or are cheaper to implement than doing it other ways, or more flexible in doing that. The main things I can see are -
Providing composite or HDMI display ( no VGA ).
Providing an LCD / touch interface ( not supported yet ).
Providing a network connection ( wired or Wi-Fi ).
Providing interfacing to a USB device.
Being a computation engine.
Being a data logger or large add-on memory.
Being an MP3 player.
Providing text to speech output.
It's just a small and fairly cheap PC in hardware terms so I suspect the hackers it will appeal to are those who will find it easier or cheaper to use than hacking desktop PC's, routers and the like to achieve things.
It's a shame it doesn't support USB device mode because that would make a neat datalogger; just plug it into a PC and it would look like a USB memory stick with the log files ready to copy and paste.
It's also unfortunate it doesn't support being powered from a PC or TV's USB's port without risking damage; the power requirements are quite demanding with some issues still to be resolved.
The biggest problems at the moment are IMO - getting hold of one, resolving power issues if they occur, needing to know what you are doing and limited help if you don't know.
what compiler is the current favourite?
Probably C/C++ at present but it's really too early to say what will be best; there's still work to be done and knowledge exposed in how to do things with the board.
What was the reason for the delivery delay?
Decided they couldn't manufacture in UK so had to find another assembler.
PCB track found to be missing, had to change PCB design.
Clock crystal chosen not readily available in quantity and had to be re-sourced.
Wrong ethernet jack fitted; had to be sent back, removed and replaced.
Belatedly discovered it needed to be CE certified before public sales could commence and having to do all that CE certification requires.
Licensees not able to start manufacturing on launch day.
Nothing that can't afflict any project.