Aperture 3.4.1 is driving me nuts too, on OSX 10.8.2 on my late 2009 MacBook Pro. It's unstable, every few days it needs to repair the library (which takes ages), and it occassionally eats up *all* of my RAM (8GB) and locks up my computer. No other application I use is even close to as unreliable. And it used to work fine before Mountain Lion.
In response to William and Kirby, I've reformatted my hard drive and resinstalled OSX (after one of the Aperture initiated crashes), I've rebuilt my Aperture library (twice), I've deleted my Aperture preferences, and I've deleted and reinstalled Aperture from the AppStore. Disk First Aid finds no problems with my machine. I'm at a loss as to what to try next (besides Lightroom).
It's great to hear that most users are not being affected by this. I'm still finding it hard to believe that the problem is not Aperture directly. Come on Apple! Show Aperture a little love.