I found one in the back stock room of a musical supply store in San Francisco in 2008. I bought it for $300 knowing it to be non-functional. I then began a two month restoration process that taught me quite a bit about the instrument.
First, many of these were placed in long term storage by their owners and were forgotten. During storage, two things tended to happen to them. First, foam padding in their stock Anvil cases melted over time and covered the wood sidings, keys, et al, with a black tar-like goo. This was an easy, though laborious, process to correct. It involved sanding down and refinishing the wood along with careful removal of the material from other surfaces with a variety of instruments.
Second, on the motherboards, memory storage is preserved by a simple battery (similar to those found in hearing aids). Due to long storage, these batteries corroded and the acid would drip onto various parts of the motherboard. Here's where the real trouble began. It all depends on how the instrument was situated in storage, if the battery acid leaked, and what components were damaged in the process.
My instrument had a faulty power supply due to a poor design by ARP which designed the synth. I had two options for repair. Pete Miller at CAE Sound in San Mateo offered a more expensive, though more authentic approach using circuitry from the era which he felt would serve the instrument better over the long term. Greg Montablano in Piedmont offered a less expensive route using modern equivalents. I opted for the Pete Miller approach. Pete did an outstanding job.
Greg helped me restore two of the non-functional voice boards. The Chroma has 8 voice boards that are individual circuit boards (gigantic by modern standards - nothing 'micro' about them!). Greg was able to quickly troubleshoot the circuit boards and had spare parts around to replace the dead components. Greg's repair was inexpensive and turn around was fast.
Once these repairs were complete, the synth ran in perfect form for over 1 1/2 years until I decided to sell it to buy other gear and to clear up space. This synth is huge in the style of early poly analogues.
The Chroma, as mentioned elsewhere, has an unbelievable keyboard manufactured at an organ factory Fender located when they took over the instrument from ARP. The builders of the instrument knew what they were doing in terms of keyboard design.
The interior circuitry was not cleverly designed, hence the requirement of restoration should you purchase one that has not been gone through.
I found the programming to be not very engaging. It simply took far too long to get a sound I was looking for as it required constant review of the programming sheets to understand the deep menu options. It is an example of what not to do when designing a sound design unit. It ushered in the era of the Yamaha DX7 and other synths that were menu driven.
The sound is very unique and is unlike any other synth I've ever heard. It can be lushly thick (some say sweet - not sure I know what they're referring to). They keyboard action makes it a most unusual playing experience. I traded sold mine for an Andromeda which I preferred in terms of hands on ability to shape sounds.