K, I've been testing this and here are my thoughts:
1) 8.5mb download and 30mb of HDD space is not what I'd call efficient. Still it's early but if they are going public they should take it. For comparison, the most bloated version of foobar2k which has about two hundred more features than Songbird is 2.5mb and only requires 9mb of HDD. BTW, it's not about absolute size, but efficiency.
2) On my 240 song test database this thing was using over 70mb of ram compared to the 12mb of foobar. Again, this will likely see improvements but I'd be surprised if they got to only double of foobar. OTOH this is very much a match for iTunes.
3) For a iTunes killer, a program that can't access the iPod (or any other device) nor the iTunes store I'd say they have their work cut out for them. According to them the first one should be resolved in a future version but the second, they say, will only be resolved when Apple allows it. Yeah, right. A iTunes killer this is not if people still need to use iTunes to buy music/videos for their iPods.
4) For those that use iTunes only as a music jukebox and have never used the online store this also won't be the iTunes killer. I mean, if you have put up with the bloated mess that is iTunes and have not used up its single exclusive and interesting feature then no matter how good, no other program is going to replace iTunes in your heart.
5) It's far too early to talk about the UI, etc. Right now it's slightly better than iTunes but that's not saying much. I'm happy that future versions won't come with hardcoded services so right now this is a bit hard to evaluate.
With the multitude of audio players out there this isn't bringing anything new to the table.