Problems with output cache

[eluser]n0xie[/eluser]
[quote author="cahva" date="1266983649"]Ok, I was a little harsh. I meant its useless on user-driven sites where content changes according to user.[/quote]
That depends entirely on your application and/or the way you set it up.

Since the output library caches based on URI, the chance of having a profile showing up from the wrong user with the url scheme domain.tld/profile/id is highly unlikely. For most other 'widget type' changeable content, AJAX is usually the way to go.

Most high traffic sites would not survive without static html caching. I'm not saying the CodeIgniter Cache is the best implementation ever, but I would hardly call it useless. Remember you can tweak the TTL of the cache making it so that pages that change a lot (say the homepage) gets refreshed often while others remain in cache for a long time.

Keep in mind that in most applications, the round trip to the database is the most expensive (i.e. the bottleneck) so anything that lowers the hit on your RDMS, will most likely dramatically improve the performance of your website.