I really like the Bernina Bernette Deco embroidery machine, the 600 and 650. I currently own 2 600's and 1 650. The only difference is the bobbin mechanism and designs built into the machine. I bought them all used and put them in my classroom.
The 650 has a faulty bobbin mechanism in models produced during a certain time frame which is cost prohibitive (in my opinion) to fix. I don't think you can know which one you have until you have to get it fixed. The 650 I currently have does not have the faulty mechanism. The 650 has probably twice the number of designs built in as the 600.
I've never had a problem with my 600's, and the built in designs are really nice and very detailed.
I bought 3 of the machines on Ebay and 1 of the machines from my local dealer. I never pay more than $300 for the 600 model and $350 for the 650 model. And I have paid less.