It's noticeable in the fact that the truck does squat under the load, but it is manageable in the sense that it doesn't cause any handling problems or anything like that. The weight distribution hitch does help quite a bit. The first couple of trips that I took were without the hitch, and there was a difference when I did start using it.
These trailers are built to cater to ATV's, which aren't really known for being lightweight, so I feel that they are more than capable of hauling dirt bikes for sure. Throwing my street bike on there might be pushing the limits of the weight rating, but it hasn't caused a problem to this point.
My FZ-1 is around 540 lbs, so you'll notice that I put it in the rearward position so that it's weight was as far back as possible. It will never be loaded in the front position as I feel that would be too close to the hitch.