I have a GA645 and like it as a carry-around big neg P&S. No, the AF is nothing special but not too bad. I must say though that as someone that shoots a Fuji 690III and a few 6x6 cameras (Hasselblad 500CM, Rolleiflex TLRs, Rolleiflex SL66) I find the negs from those bigger negs to be MUCH better in terms of smoothness of tonality and a medium format "look" vs. the 6x4.5 negs. If you really treasure compactness then IMO just use a really good 35mm camera with the lower weight and ease of smaller interchangeable lenses. For me 6x4.5 is simply not enough of an improvement in look and tonality vs. good 35mm (Leica, Contax and such).
Do not get me wrong the negs from my GA645 are nice I just do not see as much of a jump in the "wow!" factor as I do comparing 35mm vs the larger MF sizes I mentioned, and yes, I know how much bigger percentage-wise the 6x4.5 is compared to 35mm----the differences I desire and see simply does not come in until 6x6 or larger. 6x4.5 is nice, an improvement, but not over the top like 6x6 and 6x9 is.
Now all that said consider a Mamiya 7. Big 6x7 negs, compact, not heavy, and interchageable lenses. Not cheap however, especially the 7II. Amazing glass though.