What went wrong with my Chocolate Chantilly (Hervé This' recipe)
This dish should taste like the chocolate you use and have the texture of a mousse. I'm not familiar with Weiss chocolate, but I don't think 57% is adequate. I would suggest at least 70%. 

Perhaps you should try a different brand? Again, I'm not familiar with Weiss, but is it any good? Do you like the taste of the chocolate alone? You should. If you try a different brand, I'd suggest Valrhona.

Update

The graininess is a result of over whisking. If you over whisk you can simply return it to the pot and start again.

Also, I looked up Weiss chocolate with 57% cocoa solids, and I'm more confident that this is the primary cause of your flavor problems. The breakdown for this is: 57% Cocoa, 42% Sugar, 36% Fat (the fat is included in the cocoa solid). Cocoa butter (the fat) is what carries the chocolate flavor (provided by the non-fat cocoa solids) and provides the richness and body of the chocolate. Chocolates with 57% Cocoa typically contain 33-36% fat content. Whereas chocolate with 70% cocoa solids typically have a fat content in the range of 40-42%. This lack of fat can lead to a "blander" taste in the finished dish.

Another thing to keep in mind is that this dish gives you a fair amount of wiggle room. If you whip it up and it feels like it's still lacking in some fat, then melt it, add more chocolate and start over. If you end up with something that isn't light enough, you can simply start over and add a little more water. As long as you don't burn anything you can do this indefinitely.

Finally, if you feel like experimenting you can use something besides water. In the original article Herve This recommended orange juice or blackcurrant puree.

Another Update

I found a video that uses tea in place of water. It also demonstrates how to do it without whisking, using a N₂O siphon.