I have one, so I like them. Heh. My standard one is slightly wonky but as long as the bar is well-fitting you can't really notice it. If you want, get the top one repierced but I wouldn't think healing both at the same time would be too fun so it'd be a while before you get the inverse done...
You have to have a fairly pronounced lip at the bottom of your navel - your piercer might not think you're suited to it, so be prepared that you might not have it. However, I hardly have any lip at all but I could get mine done, and it healed quicker than my standard, and also didn't migrate at all (whereas my standard migrated 3mm or so in a couple of months, then settled).
It's slightly more painful than standard but neither are too bad. It's over quickly, heh.