What was your use case, and what was your experience with it?
How many string keys? Millions? Billions? What happens if you lose an update?
Redis great for lots of rapid reads and moderate write speed for data that fits on a single server or can be manually sharded well (if it's straight k=>v, as you describe, that basically means your json fits in RAM; if you're using larger objects like sets/zsets/etc, it becomes a slightly different discussion), as long as your application can lose a few seconds without killing you (BGSAVE isn't instantaneous, of course).
http://redis.io/topics/twitter-clone
I want to use it in a small project so I can learn the design patterns associated with key-value stores. I'm sure there are good and bad things about using Redis vs Postgres.
But it has to hold my data, so... does it hold my data or not? I'm talking about single box stuff here, of course.
(Of course, I'll just use it and find out myself).
Is all data in a database that important? Maybe alternative databses would generate new webapp patterns, like dynamic languages do.