[rbolkey]

As a corollary, for what should I avoid using Redis?

I've heard wonderful things, but I've also heard that you can easily abuse Redis for things that aren't appropriate.

[kcorbitt]

Data too large to fit in memory; anything that you're going to want to query relationally (as opposed to just key/value) in the future.

[sitkack]

Not entirely true. Redis sets are quite powerful.

[yareally]

When you find yourself doing things too often you should be doing in a relational db (i.e. lots of foreign key relations).