I have to admit that I thought that the Brita filters were only activated carbon filters. Didn't know that they also include a cation exchange resin.
I see that the cation exchange resin is charged with potassium instead of sodium which should reduce its impact to brewers.
I'm a little surprised by the reduction in total dissolved solids. If it were just the reaction from the cation exchange resin, the TDS would remain similar. But, it appears that the filter also decarbonates the water as it passes through. That causes the Ca and CO3 to precipitate onto the activated carbon surface. That is the reduction in temporary hardness that is touted by Brita.
I wouldn't call the filter worthwhile, but it doesn't really hurt the brewing water. Go for it.