I tried out the mid priced one with the Ultegra group set. This was a few months ago, I think it was around the £4000 mark. I liked it a lot, but the motor was not powerful enough for me up the many steep hills I have in my area. Although the motor is supposed to be rated at I think 65 Nm or similar, my 50 Nm Bosch motor on my Cannondale Neo was much better at getting me up steep hills.
Now I admit this may be down to my riding style, the shop told me to get the best out of the Fazua motor on the hills one really needs to be out of the saddle and really stamping on the pedals. Really going for it was how they told me. Now this is not how I can ride up hills anymore, I tend to put it in a really low gear and spin a fairly high cadence. This did not seem to work too well with the Fazua but works great with the Bosch.
If you intend on riding up some fairly steep hills (greater than 20%) then I suggest you try out the Cube on them before you part with any money.