-I've been to ITAMAE (the one in Palermo Viejo) twice. The second time, I chose not to eat. http://www.itamaesushi.com.ar/ Their website looks good, but in person... not so much. It's a chain that specializes in an "All You Can Eat" option. AR$65/AR$75/AR$90 levels
I think that Osaka (the corner of Soler & Fitz Roy in Palermo Hollywood) is really good and has sushi as well as non-sushi dishes. It's officially "Peruvian & Sushi".
Also, there's The Little Rose in Palermo Viejo (Armenia 1672). http://www.alifewortheating.com/ba/little-rose/ I have some friends that go there and like it. I've never been, but they know that all sushi isn't supposed to come in rolls with Philadelphia Cream Cheese. (ITAMAE doesn't quite understand this.)
there is a great restaurant in Begrano, a few blocks from cabildo and juramento which does pasta and sushi, weird combination i know but that's ba for you and all it's eccentricity. when i was last there we ate 'italian' but i hear great things about the sushi, and the sushi chef is japanese, guess that makes a difference. i only ate takeout dashi and ate in itamae both of which i thought were acceptable, although dashi over expensive. the prices on the website for ieri look reasonable, you might want to give them a go and let me know, i'm moving back out in november. http://ieripasta.com/sushi.html
you should check out the big grocery store on Mendoza somewhere between the china town and Libertador... they have great fish and supplies to make your own sushi! at least this way you´ll know maaas o menos where your ¨raw¨ food is coming from!
We checked out Gaijin (Bulnes y Paraguay) on Saturday, and I have to say it was pretty good. Lots of salmon, but I did also get one fresh piece of tuna (why is it so hard to get tuna here?). The price seemed to be pretty reasonable too.