I have the Jem 760 Platinum and I've had it for about four years. I haven't had any problems with it mechanically. I've hauled it all over to different classes and it's been on a lot of road trips with me. Basically, it does it's job pretty well and I've pieced a lot of tops on it when I've been out of town.
I am a little dissapointed with some parts of it. I took it to class on FMQ. We practiced FMQ techniques on small squares and it just did not perform well. It might just be my machine, but it felt that the motor just wasn't quite robust enough for FMQ. At home, I use my bigger Janome if I want to FMQ and this class was my first try with FMQing on the Jem.
Oh-- the cost-- my Jem cost about $450 when I bought it.
One thing to watch out for on the Jems -- A friend of mine bought a Jem at Hancocks (not sure what flavor of Jem it is)and it did not have any kind of light in the machine. So she had to buy an external light to attach it to the machine. So regardless of what you buy, check it out carefully.