I like all of their films, even the Delta 100 and 400, which I don't like!
With a 400 film, you can get usable exposures with film ratings of 1600 and 3200 with any standard or "hot" developer. Ilfotec HC, ID-11, D-76, HC-110, etc. should push the highlights just fine, but it will be easier with harsher things like DD-X, T-Max, X-Tol, Microphen, etc., and these will also probably give slightly brighter tones on the low end.
I shoot Delta 3200 quite a lot at the very edge of its usability, so I would like to give a runthough of what it is like to use it this way.
Delta 3200 is an ISO 1000 film. (You should read the data sheet for all these films before you try them, IMO.) Don't forget that when trying to figure out how to expose and develop it. For example, uprating it to 6400 is not a one stop underexposure, but a 2-2/3 stop underexposure.
However, like the other Deltas, it can fairly easily handle one stop of underexposure without requiring increased development to compensate. So that gives you an extra stop of underexposure latitude, if you need it. (Also interesting to note is that Delta and T-Max negs look about a stop thinner than a conventionally grained film, even when properly exposed and developed.)
For much of what I shoot (live bands and people in small local clubs), I have to pretend that I am shooting an 8,000 to 16,000 film. In better-lit venues, I am fine with a 400 film, pushed if need be, but I have to underexpose the hell out of Delta 1000 when shooting these smaller places.
For instance, I start with 1000, since that is the speed the film *actually* is. I take a spot reading with my Pentax meter off of somebody's face. Depending on the light, this ranges from EV 1 to EV 3 usually. My spot meter tells me to make an EV 1 middle grey, I need to shoot at f/1.4 at '10 sec. That ain't kosher for shooting musicians (if a sharp shot is the goal), so I say, "Shit this light sucks, but nothing in the darkroom can make up for a fast shutter speed now", so I shoot the shots at '125 when they are moving and '60 if they are more still, knowing they are going to be dreadfully underexposed no matter what I do. If I can brace myself on something, I will shoot at '30. (This is all with 50 or 55mm lenses, BTW.)
Shooting at '125, you have a 3-2/3 stop underexposure, which places the guy's or gal's face on zone I-1/3. To have it be a zone V instead, I would need to be using a 12,800 film. No matter what, though, it is a 1000 film. Always remember that. Any other rating than the true ISO of 1000 is simply a change in the amount of exposure, not actual film speed.
The problem is that something placed on zone 1-1/3 is almost impossible to push any higher with a standard soup, as it is practically blacnk and holds almost no detail when printed normally. Much can be done in development and printing however.
Standard soup will have a very hard time getting anything more out of that zone I-1/3 placement. So, in these cases that are on the absolute edge of usability, I depend on D-19, personally. It is an X-ray developer that fogs the hell out of your film, giving it "fake" speed which helps raise the shadow values on the print, and also increases its contrast to the max., which helps a little with the muddiness in the low tones when you go to print. I use it stock strength, usually for at least 15 minutes plus a 5 minute stand. (I have not ever extensively used those other hot developers I mentioned, and they might work fine as well.)
Anyhow, just the "fake" speed increase due to the increased fogging gets the zone I-1/3 placement up to around what will be a zone II density with normal printing. That's a lot of fog, but I want it there. That's why I develop it long and let it stand at the end. The contrast enhancement adds slightly more, as does intensification and toning if needed. In the end, you can get that density to be approaching the density that corresponds to a zone III with normal printing. Then, you have that easy 1 stop of underexposure latitude of the Delta/T-Max films to save your ass in printing, which means that even a poor printer would be able to get the performer's face to almost a zone IV print value. Then, with difficult printing on harder papers and/or or using infectious developers, it is a printable picture in which you can get that face up to at least a zone VI. There is loss of detail, lots of contrast, and lots of grain, but it's better than nothing.
Like I said, light this low that is also low in contrast puts this film at the edge of its usability. However, even there, there are ways to get a certain kind of picture if you work really hard at it. Imagine what you could do with it if you only had to underexpose it by one or two stops, and had some more contrast at the original scene?
I love this film so much because it is so bulletproof. You have to be trying really hard in order to not get printable pix with it.
Sorry to bore the hell out of you.