Most of the time, neither "this" nor "I agree" are of value on HN. If you have something unique to add to the conversation, say it, but don't waste space letting us know that you agree. As philwelch said elsewhere, your agreement isn't valuable information.

There are only a couple of exceptions I can think of:

- if someone says something about your business, personal life, or recognized area of expertise, your confirmation is of value. In that case, "I agree" is a perfectly fine standalone response.

- if you're responding in disagreement to one part of a post, it can be helpful to mention that you agree with other parts, in order to stop responses like "you ignored the rest of what I said". Clarifying the scope of your response can help keep discussion focused, and is therefore of value.