niche apps — Open, #open, Plura... anyone tried these?

keep seeing ads for these smaller apps specifically built for poly/enm/swinger communities. Open (formerly #open) and Plura come up a lot. has anyone in the uk or anywhere outside the us actually tried them? the marketing makes them sound great but i suspect the user bases are tiny outside major us cities. is it worth the $10-15 a month subscription if i'm in like... bristol or edinburgh? or am i better off paying for feeld majestic instead...