Having just found the iOS app GoQuest (which offers mainly 9x9 games), I'm curious what the general consensus is regarding the usefulness of 9x9 games as a means of improvement. I know smaller boards are incredibly useful for beginners because they eliminate some of the complications and reduce the importance of difficult-to-judge concepts like direction of play, but once someone is into the single-digit-kyu range, do you think it's useful to keep playing 9x9?

One way I've thought about looking at it is that 9x9 is closer to tsumego practice. With a smaller board, it's more important to read long sequences that look more like middle-game fighting on 19x19 boards and tsumego problems in general.

Other thoughts?

Nine by nine go can be worth time for anyone. Pros in Japan play it. For amateurs the main benefits would be reading practice. Other benefits are sente-gote issues, for example losing sente when you shouldn't is a bigger mistake than on the 19 x 19.

If I have a spare five minutes I will often play a 9x9 against a computer. Unless I am careless I will usually win, but that is not the point. I am exposed to tsemugo positions which I may only see once a year otherwise.

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Still officially AGA 5d but I play so irregularly these days that I am probably only 3d or 4d over the board (but hopefully still 5d in terms of knowledge, theory and the ability to contribute).

I find that 9x9 is great practice for ko, because the game often hangs on a ko and there tend to be big ko threats.

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Revisiting Go - Study Journal
My Programming Blog - About the evolution of my go bot.

I went from complete beginner to somewhere around 15k by playing hundreds games of 9x9 against the computer. Its definitely very useful for beginners.

For more experienced players I'm not sure how helpful it is, but it does seem useful to me for practicing the endgame. On the smaller board there are only so many possible moves, so you can try to calculate the value of each, and the doublesente/sente/gote status of each, and the play the correct move.

What people said, also GoQuest have the adventage of beeing easy to access and have quick games with people (well if you use the setting to avoid playing bots)

From my beginnery stage it also demand you learn to read quickly. (Which for me might be keep in bad habits though)

One of the greatest things about 9x9 games is that they don't take hardly any time. Most of mine are done in 5-10 minutes. It is a great way to keep up your go skills when you don't have the time or aren't motivated enough to spend an hour or more on a 19x19 game.