Hey Chamai,
I never tried the D1280, but I do remember trying the D640 when they were a current item in the stores back in the day. I remember not being too impressed with the sound because it seemed to have a somewhat dark quality to it. At least to my ears it seemed that way, but remember, taste is subjective.
It's the predecessor to the 1.28i, which followed the D series delays. I like the "i" series as well as the "S" series delays because they do sound good. The 1.28i has a similar layout as the D1280, with a selectable waveform added, so I can choose between Sine, Triangle and Square waveforms.
All I can say to you is go audition it. try out a few different effects settings from the manual, and see what you think. If it sounds good to you, and the price is right, then go for it. In the early 80's, this was considered state of the art studio processing gear. By today's standards, it would be considered a mediocre pedal quality device to be honest, just like all my ADA effects processors.
Just like all the old ADA gear, it's all about the character of tone which I think is quite unique to ADA. (Just my opinion). All of the ADA processors have their own characteristic personalities if you will. Who knows? You might find a dedicated effect situation for it down the road. I certainly have for many of my "S" series delays and my "i" series delays.
Just my personal thoughts on it.
Harley