Somebody I know was working in Riga for a while and took Russian lessons at the time; I don't think he learned very much though! I never asked him why he took Russian lessons and not lessons in Latvian - maybe he was just being pragmatic about the international usefulness of Russian vs Latvian.
I have heard that about half of the population in Riga are currently native Russian speakers.

I have been in Latvia but it was a long time ago. I remember seeing signs in cyrillic lettters (Russian!) everywhere - they stood out to me, as I could not read them (was quite young). As you probably know, Latvian is not written with cyrillic letters. My dad was making comments to the extent that Latvia was being "russified". I think there has been a big backlash against that after Latvia became independent, and now it's exactly the opposite way around; Latvians are wanting to assert their nationality and the Russians there are being "latvified", quite deliberately, in a rather insensitive way.

I too was briefly thinking that it might be pretty clever idea to spend a bit of time in Latvia studying Russian, since I wouldn't have any visa trouble whatsoever, as Latvia is in the EU. Also, there is a super handy ferry straight from Stockholm where I come from, to Riga.

I even asked about the idea here in this forum, and a girl who is a native Russian speaker living in Latvia responded. She was essentially saying that the idea of studying Russian in Latvia was a pretty bad one. According to her, the areas of Latvia that are fully Russian speaking are not very nice places to study in; not interesting at all. She was also saying something along the lines that the Russian speakers in Latvia don't always speak very good Russian. Of course, that's her subjective view, but she was quite adamant and after reading her comment I wrote off the whole idea.
A visa is not such a big problem really - and studying Russian in Latvia is probably a rather poor substitute for doing it in Russia.

But if, like my friend, you happen to be there anyway, then I guess it's one of the best places in the EU to study Russian..