Hey David,
Welcome to the Forum
I already own all of the delays mentioned here and I can give you a brief review of each one.
2FX- is a pretty good delay in the fact that you can run two effects at the same time if you want to. I like it for the doubler, flange, chorus, and some of the subtle long delays. The only con I can come up with as far as this unit goes, is that it seems to be a bit lower fidelity than the S-1000, any of the "i" series delays, and the Digitizer 4.
.64i, 1.28i, & 2.56i - These are great sounding 1/2 second, 1 second, and 2 second delays, (respectively). Very similar in function and performance to the S-1000's but with more control over the modulation section than the S-1000, and Digitizer 4. They do a good doubler, flange, chorus and delay effect to perfection. easy to dial in and they can even do a decent imitation of a Leslie type of effect. They have a very musical quality to them.
S-1000 - One second delay, and THEN some. I may sound a little biased, but this was my 1st ADA rack delay, and I squeezed every possible effect I could get out of it with great results. I was able to get similar chorus effects to the TC Electronic 1210, and the flange effect too. I was imitating EVH's delays that he got from his Lexicon PCM 70 with it, and thick and lush chorus, and Leslie chorus effects.
The only downside to all of the effects I have listed so far, is that you set them to the effect you want, and that's what you've got, (with the exception of the 2FX). Basically, they are one effect per unit.
Digitizer 4 - Programmable one second digital delay. This is the last delay effect processor ADA made before they discontinued all of their effects processors, and this one resides in the group that includes the Pitchtraq, and the MQ-1 Programmable EQ. After these were discontinued, ADA focused on their preamps, power amps, and speaker systems.
The D4 could do a lot of the same things it's predecessors did, but it could no longer do some of the smoother effects that required a sine wave, like the Leslie or smooth chorus type of effect. It can give you some very strong flange and delay effects that you can save to one of 16 memory slots, but you need the DS-4 footswitch to recall them remotely. They are not MIDI capable. What's really nice about these delays, (all of them), is you can split your signal to have one channel with the effect, and one channel dry to blend in the effects with your preamp/amp tone unaffected. Overall, any of these delays would great in a rack rig.
The only trick now is to find one that is in good working condition. They are old, and many on the market have issues. Ask the right questions before pulling the trigger would be my advice.
I hope this was helpful.
Harley