The configuration of the Yamaha Electone EL-90 will take a little while to understand and isn't the most user friendly keyboard. Around this time period when the switch was being made from analog to digital, a lot of instruments and other gear were hard to use because no one was used to the interfaces and navigating through the different digital screens and whatnot wasn't too easy to do. While this keyboard certainly has that influence on it, after some use it becomes easier to do everything. There are a lot of different features on this keyboard and having the manual around is necessary if you are unfamiliar with this instrument.
The sounds on the Yamaha Electone EL-90 are very versatile as it is capable of getting all sorts of unique sounds and emulations. Its vast array of parameters and digital storing and settings make it possible to achieve an almost unlimited amount of different tones. While some of the sounds aren't as realistic sounding as modern instruments, this certainly has a unique sound to it and would encourage those interested to try it out and see if it is something that they are into and is something that will fit with your own unique style.
I've been using the Yamaha Electone EL-90 for about two years and it is definitely one of the most interesting and unique Electones. It has a ton of different types of keyboard sounds and is the blueprint for modern digital keyboards that Yamaha makes. These are pretty rare to find these days and I don't think they are too cheap if you can find them, as they are huge instruments and hard to come by. While the sounds on this keyboard are certainly good and interesting for creative purposes, if you are simply looking for a digital keyboard I would advise to look elsewhere. This is a great unit but unless you are after this one specially I would recommend trying out a newer digital keyboard as they sound better and are far smaller!