Has anyone ever tried it yet? What flavor/mouthfeel/extraction benefits are there, if any?
If you heat the water as per most brewing methods, it would no longer be carbonated/sparkling by the time you brewed with it.
You would thus be confined to a cold-brew method, and if you wanted the water to remain carbonated you'd have to cold brew in a sealed container. That would be moderately inconvenient but should be possible.
Or you could brew coffee by any means and force carbonate it after the fact, which would thus have no effect on the brewing.
Never tried it, don't find the idea personally interesting, but all it should cost you to try would be a bottle of sparking water and the hassle of getting the coffee grounds into it, waiting a day or so, and then pouring it out through a filter and cleaning up the mess before recycling the bottle. Please report your results.
Good coffee is extracted via highly measured methods, methods that attempt to always replicate uniform extraction.
Blooming is a step taken in drip/pourover and sometimes full immersion methods to help coffee grounds degas before the proper extraction. This is because as gas leaves the coffee, it cannot simultaneously absorb water.
Thus, using sparkling water is heavily detrimental and it introduces a large amount of bubbling which reduces actual coffee-to-water contact and result with an poorly extracted cup born from an inconsistent extraction.
The other issue you'll run into is that sparkling water is actually pretty acidic, and that's going to change the overall extraction process as well.
If you want sparkling coffee, make cold brew and then put it in something like a Sodastream to force-carbonate it.
I tried it this weekend. The Nespresso machine sputtered and wheezed a bit, then shut down with blinking lights, and the pod inside had the foil blown off and there were grounds in the hopper. Bad idea.