i'm a Janome fan. i have two ... one of which is the 6600.
the accufeed mechanism has never gotten in my way. there are only two things i don't like about it.
(1) the acufeed feet are, indeed, a pain to change. not as in painful. as in nowhere near as convenient as snap on feet are.
(2) about halfway through a bobbin, the top thread will pop out of the arm thingie and it jams. it takes less than 30 seconds to fix but is a bit annoying. i had it for six months before it started doing that. my friend's has been doing it since the beginning. what i can't figure out is whether it's the machine or the way i wind my bobbins. (i bought a winder, so don't use the sewing machine to wind them.)
if you can afford the Horizon, i'd get that because of the extra throat space.
if you're considering the Horizon, you might want to check with a Pfaff dealer first. the Quilt Expression 4.0 is their equivalent to the Horizon for around the same price. the Pfaff version of acufeed feet snap on. i don't know if it comes with an extension table like the Horizon does.
as much as i love my Janomes, i will look at the Pfaff first if and when enough $$$ show up in my wallet to upgrade.