I had long dreamed of owning the GF670 some day but honestly I didn't think I could ever afford it. When I saw the GS645 for a reasonable price I thought to myself, "This is the next best thing!" After all, there are very few "modern" folders out there. So I picked it up, found it to be rather quirky, but managed to get used to it, and loved the images I got from it. Then one thing led to another, I received a bit of birthday money, decided to sell a bunch of nice cameras that I wasn't using and pretty soon I had enough in my pocket to get a used GF670! I imagine my prior experience with the GS645 made it easier to get used to the GF670, but I do find the latter camera to be less quirky. I suppose the native portrait orientation is the weirdest thing about the GS645. Also, I find the focus tab and aperture ring easier to find and operate on the GF670. Maybe I just didn't spend enough time with the GS645, but I could never find the aperture ring while looking through the VF and even struggled to find the focus lever. The viewfinder on the GF670 is much larger and brighter than the GS645. Furthermore, the option of going 6x6 or 6x7 is very useful for me. I frequently switch back and forth, most often using 6x7 for landscape and 6x6 for other handheld work.
And even though it may just be my imagination, and as excellent as the lens on the GS645 is, I find the lens on the GF670 to be nothing short of stunning at times. And when you think about it, the GS645 came out in 1983, exactly 30 years ago. The GF670 was released in 2009 and is still in production. It should have the advantage of ever advancing technology. The only thing I wish the GF670 had was a timer shutter release like the GS645 does. One weird thing about the GF670 is that the shutter release is ridiculously quiet; so quiet in fact that you won't even hear it if you are on a busy street.
So while I haven't had the GF670 long enough to comment on reliability or relative sturdiness, I do find the ergonomics of the GF670 preferable, mainly because I don't have to rotate it to shoot landscape orientation. And yet, I do prefer the larger negatives of the 6x6 and 6x7 format. If cost is a big factor the GS645 won't disappoint you. But if you can afford it I know you will absolutely love the GF670.