Have you ever played matchmaker and introduced a couple to each other? If so, how did it work out? If not, why not?

Several times, when requested to.
Only one match worked, but it was spectacularly successful; the pair are still together and happy after over 30 years.

Ari and I were match-made by my then flatmate and friend, Madeleine.
We were very diffident about getting together, broke up three times, eventually married after 19 years courtship, and now one could call it a companionate marriage that works well enough most of the time.
Still needs work and patience to make it work. There are times when energy for it fails.
But Ari is a very good friend - for that alone he is worth everything.

I accidentally connected 2 couples, who are still married after 30 years. Both couples have 2 kiddos. 

I was in a matchmaker situation.  My best friend brought over a guy she through I would like.
He was short and good looking...however....oh... no thank you very much. hahahaha
But guess what...she ended up marrying him.