I started out with Audacity because it was the easiest to get working on Linux. That was sufficient for years, and it still is for 2 or 4 track work. Now I've started multitracking with 20+ tracks, and I use either Reaper or ProTools 8 on Windoze.
The thing I most like about Reaper is that the keyboard shortcuts are more configurable, and so I can use the same keystrokes I've been using for years. You can "Render" the project to disk as fast as the computer can go. So I can render a 2 hour show in 15 minutes, and go listen in the other room. There are a lot of free plugins, it's on the edge of the "free and open source software" mindset, I could write my own plugins if I wanted. The way the plugins are organized is a bit of a hassle, but it's livable (opening up a bunch of individual FX windows, as opposed to Protools where you go to Mix mode and see everything at a glance).
I have a real love/hate relationship with Protools. I do love the Mixer window with the plugins/routing on one screen, but I hate the keystrokes (Zoom in is "Control + ]" WTF?) Also hate the real time rendering and hate the fact that I get a lot of stupid interrupts. Protools really wants to be the only thing running on your computer. The mindset behind any additional tools is MONEY, MONEY, MONEY.