$650 for a JV90? That aint bad at all. Considering that here in New York City, a used JV80 goes for about the same. Usually $695 infact.
The JV90 is a great board. You have 76 semi-wieghted keys and the option to install both an SRJV80 expansion board and a SVJ Voice expansion board. As for the SRJV, you have 18 choices on the expansion sounds you want. For the SVJ, you have about four. The lastest VJ option, the VJGS1 is great as it adds 1,117 new up to date voices. The JV90 also makes for a great controller with its right sliders. On the down side, you get 56 voices of polyphony only with the VEJV1 board, otherwise you are stuck with only 28 voices.
However, if you want more power, a used XP50 goes for about $850. It has more sounds and will take 4 expansion boards. A used XP80 goes usually for $1350 and a used XP60 at $1100. The 60/80 have a larger, more detailed screen than the JV80/90/1000 and XP50.
The JV1000 came out at the same time as the JV90 and has the same, but moe features like 512 voices, more waveforms and an eight track sequencer. There are TWO screens on this keyboard which is very cool. One is for the sound engine and the other is JUST FOR the sequencer.
Look around on the web. A used JV1000 should go for around $950-$1300 depending on where you look. Good luck.
The Infamous EPU.