I used Counter Chansey in DP UU. I found it worked pretty well to be honest, although it was total cringe (for me), when I used Counter and the opponent missed, or didn't attack.
However, Counter Chansey was great back then for Dugtrio, as Dugtrio couldn't ever OHKO Chansey, but with Dugtrio's tiny HP, the fact that it never used non-attacking moves, and Chansey's enourmous HP and minimal Defense meant that Chansey always OHKO'd Dugtrio. I guess the only time Chansey would've failed would've been if Earthquake got a critical hit. I just always found it funny that Dugtrio was like the instant switch-in to Chansey, but my Chansey always beat it. :D I suppose if Counter Chansey was more common, people wouldn't have used Dugtrio to beat it though.