It definitely helps ranking on search engines... but the degree to which it helps, I think is linked to how much SEO is being done for that "keyword" in any given "language market".
e.g. In Korea I have seen a parked domain rank #18 (and it only has 2 words on the parked page, that match the IDN keyword - basically no text at all otherwise).
Admitedly at the same time, the other sites for that "keyword" are doing a pretty bad job for off-page SEO (and even on-page) - but since the competition is very large for that keyword, the fact that even a page 2, #18 position, is possible for a 1 page IDN website, with a handful of words only... means their is a definite effect).
i.e. Korea is quite weak in terms of doing any real SEO (generally speaking), so the effect is obviously quite high from using an IDN keyword.
As others say, it definitely helps to own a good keyword IDN, but to what degree it helps, I think ultimately depends on how well, your competitors for any keyword, use SEO as well.
However, one thing I personally feel, is that if someone does an equally good job as their competitors i.e. everyone has equally as good SEO (and you own that keyword IDN), then their competitors can actually never beat you... as whatever that small, or more than small difference is, it will push you that step up and into #1 position (of course it means you have to equal other SEO factors).
So perhaps aim to do the best SEO you can, while at the same time owning / using the best keyword IDN (but don't think the keyword IDN alone, will get you into top position).
Cheers, Asiaplay
PS: there has been discussion over the years that the "factoring" (advantage) in the google formula, was much, much better for IDN keywords verse ascii keyword domains (however this effect seems to be lessening over time in some markets, but is still very evident in others...).
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