You'll have to bear with me a bit as I only really used this for some amateur and experimental use. I've never used this for anything professional. Soundtrack Pro is a an application that's meant to go hand-in-hand with Final Cut Pro. This allows you to do all your video editing in Final Cut Pro and all of your audio editing/splicing in Soundtrack Pro 2. By splitting the applications up like this, you have more control over what you're looking to accomplish without having to clutter up everything in a single program. Soundtrack Pro 2 is an application for adding sound to various video that you import from your FCP projects. It's extremely easy to use, but it can be a bit daunting at first. I recommend trying to check out some Youtube tutorials as they'll help immensely with what's going on and how to use this application to its fullest.
As with all Apple products, they're rock solid. I haven't experienced a single crash with Waveburner, and that's part of the magic of OS X. You're dealing with one of the best and most stable OSes out there. Everything is optimized for your hardware (assuming you're not using some sort of Hackintosh), and everything meshes like it should. The program doesn't use up too much memory, but it is a pro app, so it'll use more memory than certain other apps out there. I've been using this for as long as I've been using Logic, so it would be years and years, I guess.
I'm not a big user of Soundtrack Pro as I'm more of an audio engineer than someone who works on audio for videos, but the times I did use this program, it has worked flawlessly. There is a bit of a learning curve as to how everything operates and what everything does, but once you check out some tutorials and read the built-in help, you should be good to go.