Upload & Sell: Off
I use an Acratech GP-S with a 100-400 on a 60D and 5DII. I am mainly a landscape shooter, so I got the GP-S primarily for shorter lenses and panorama setup. I used it inverted, so that the panning base is on top; doing that saves the weight of a separate rotator for the pano rig.
Because I'm a casual, rather than dedicated bird and wildlife shooter, I can't justify a full gimbal setup, and because I hike I wouldn't want the extra weight. So the ability to use the GP-S as a rudimentary gimbal is just about right for my current needs. I'm not under any illusions that it truly substitutes for a gimbal, or perhaps not even a Sidekick, but it does perform reasonably well. It doesn't truly balance (though the lens foot plate on the 100-400 provides some adjustment), but it also doesn't droop; there is just enough friction to keep it in position, even with the lens pulled out to 400 mm. The action is still smooth (a hallmark of the Acratech heads in general) an it is effective for BIF panning, etc. (though I'm still not very good at it, but can't blame the head for that). One thing to note is that with the camera and lens flopped to the side there is substantial off-center weight. Not enough to tip over a decent tripod, but noticeable when panning quickly. Also, when the camera is rotated to the horizontal position, there is little space between the camera grip and the head, barely enough for my small fingers, so it is kind of a pain to use in this position.
It works fine for macro; probably not different from any other ballhead in this application.
So if your priorities and needs are similar to mine, you might find the Acratech to be about right, but if birds are a higher priority you might want to hold out for a true gimbal.