I cannot really comment A500/A550 wise, I have not held the bodies.
But as a KM/Sony user, I can tell you the magnification is 0.80x, that is a bit lower than the 0.83 of the non LV entry level model (and previous ones A100/km5d etc)
Though the brightness might be different.
0.80x is similar to the D60, 450d etc etc, it's a small size for a VF (easily beaten with budget 35mm models!), though not uncommon lower end. I think the point here is, that what might cut it super budget end, probably does not at a more advanced amateur level.
Build wise again, cannot say..but this is something that has gone of for a while now..cheaper materials, less appealing finish. My Km5d is unique is some respects being a lower point camera, but very well built, and better than other equivalent bodies I have used, solid and well made. Things change over the years, more often than not for the worst.
One point I will pick out, these models are to me Sony's D5000 equivalent (minus video of course!) Consumer based, and not really shall we say D90 more enthusiast looking for something more camera, less fluff. A lot of us are a bit taken aback that Sony has left out Mirror lock up, DMF (MF after focus confirm, very handy), removed the spot metering indication in the viewfinder,no AF assist in body, still no ISO on permanent display in the VF. So they are not really that well featured either. The sensor is probably good, and faster FPS welcome, but traditional Minolta users won't be too impressed, that a 4 year old budget KM has in many ways better features than a brand new model in 2009.
Sony is IMO on a full frame crusade, APS is pretty weak, at the moment anyway.
I just want to add, I might disagree with Michael on a few things, but I read his mirror lock up article, and I 100% agree with it. It is one of the most useful practical features a camera can have, and I feel strongly every model should have it, from top to bottom, in some form or another, it's almost criminal to leave it out.