This will give attitude and hdg info only.
I've heard it said that V/S will be lost, but I've turned the IRU's to ATT on the ground on a 747-400 and the V/s display looked normal. Has anyone switched all three IRUs to ATT in the air?
You say "hdg info", but heading info is limited to the accuracy of pilot input.
What else does the IRS output (with the assistance of other systems)? Groundspeed (non-GPS derived), wind, drift, Para Visual Display, track... Map displays will be affected (if Track Up oriented), affecting TCAS displays, EGPWS, etc. The Collins Multiscan Radar appears to need full IRU alignment from the tests I've done on the ground.
Of course, results may vary depending on aircraft. Also, displayed data may vary depending on how many IRU's are in ATT mode. e.g. I recall that pilot heading entry wasn't required for, say, a 747-400's Captain's displays, if the L IRU was in ATT mode, because the heading data was automatically sourced from another IRU.