I just bought a 750QE (Super Bowl weekend) and now they are coming out with a new 7 series line. They will be the 740, 770, and 790. I joined the Yahoo group for owners of the 7 series and people talk a lot about problems they are having with their machines. Many say the stitches are wonky. I started really looking at the stitches on my latest project and they are off a bit. It's not a huge, big deal, but it's not perfect. I expected perfection from Bernina. I'm pretty upset that my dealer didn't mention that a new machine was coming out. I would wait and try out both the 750 and the 770 and see which one fits you best. That is if they are going to still carry the old 7 series. You might be able to get a really good deal on a 750.
All that being said, I have made one baby quilt on my 750 and it was so much nicer to sew with than my old Viking Lily. I love the built in walking foot, thread cutter, and auto needle threader (since I now can't see the eye very well). The stitches are very strong but still easy to rip out mistakes, something that I do a lot of. One of the best things is the large harp space. The leg lift is hard to get use to and I had a sore thigh muscle for about a week, haha, but now I love it.