What restaurant is this?

I see the sign all over the place on my google maps (for example, there's one halfway up Center Gai in Shibuya on the left. The photos in Google street view give nothing away, but the logo seems to be a large red circle with a small blue and a small yellow circle inside it.

What does it sell and is it any good?

I think you are referring to the Matsuya gyudon chain store:
http://www.japan-guide.com/e/e2345.html

Strange thing is that's what I thought it was (reading it from a sign in a different area) but a quick google could only find the Matsuya depato in Ginza.

Anyway, would you recommend it for a quick lunch?

Definitely! Matsuya is good, cheap, fast and has a pretty good menu. They often have English menus as well if you need it.

Tks Smoke