Memcached was a savior for caching when it launched. I love that it was created in 2003 by Brad Fitzpatrick for LiveJournal. Each post on a users feed could have different access restrictions, and this allowed posts (or entire pages) to be cached.
I used it with Ruby on Rails for many years. It sped up pages, and just worked.
The downside (and upside for speed) is (and always was) that cache was saved in memory not disk. This meant hosting would be expensive if you have a large scale site with a wide amount of data to cache.
Solid cache has been a savior for those cases for me. We have over 100gb of cache for a project I'm working on, and it's stored in postgres on disk, with fast lookups with an index and expirations that happen automatically in Rails to delete those rows.
If I had a smaller cache need and was already using Redis, I'd probably just use that. But if speed was the number 1 factor, and I'd try benchmarking Memcached vs Redis.