I've used it at 3200 and 6400. I now dev it in DD-X, the grain is fairly large and there is not a particulary big spread of tones. I've only pushed films like T-max and tri-x one stop so I can't compare. I have been happy enougth with Delta 3200 prints and it's certainly better than nothing or crude flash lighting.
A word of caution though, the times on the Ilford site for Delta 3200 and DD-X seem way out, almost a stop out. I dev for 11minutes 30 seconds at 3200asa, that's almost Ilford time for 6400.