I have the Janome 6600 - the features are the same as the 7700 but the 6600 has a smaller harp space.
Features I DO NOT use - Needle threader, knee lift, single hole stitch plate
Features I love and would not do without if I had to replace the machine - Accufeed (built in walking foot), thread cutter, needle up/down with the push of a button, small increment needle left/right positioning, large harp size (but I wouldn't object to even larger harp!!)
Like the others above, I piece with my Accufeed all the time - but I use the accufeed foot with the 1/4" guide and I move my needle position to the right for a scant 1/4" seam - which is the only reason I don't use the single hole stitch plate.
The few times I tried the knee lift I still found myself moving my right hand into position to lift the foot ... so why bother with that dumb rod in between my legs!!
Never used the needle threader, honestly don't know how it works
And yes I would buy another Janome again ... drooling over their new one which has a larger harp area and is also an embroidery machine. The dealer gave me a SWEET deal on one but I just can't justify spending $8K on a sewing machine.