Stas - I don't think these names are worth much (certainly not more than .tm or .io). There are 4 reasons for this.
1). First, the web as we know was developed and popularized well after the USSR collapse in 1991. Hence, from the beginning it was almost always .ru (not .su). You know better than anyone else here that Russian web is called Runet (not Sunet). The .ru has been engrained in the Russian public consciousness, even more so than .com
2). Second, majority of Russian youth (i.e. 12-23) - the most active users of the web - unfortunately are not good with history. There is enough sociological literature out there, showing that they don't know who Lenin was, or what the heck is October/November 7. And they certainly will have a hard time remembering that Russia at one point was CCCP, and that CCCP in English is USSR.
3). Third, .su is hard to pronounce (i.e. from what I've seen travelling there, Russians often confuse "s" with "c" over the phone). Probably even worse, unless you pronounce .su as in "s-u" (each letter individually), "su" is too close to Russian "piss". I can already see jokes about this. Not good.
4). Then, of course, there is a problem of .su being to close to .us (Russia and the US are not exactly "buddies", and anti-American sentiment is rather high there).
Having said this, may be I'm looking too deeply into this issue and need to go back to my school books and brush up on your country's history.
Hope this helps. Good luck.