That depends on what you're looking to do that Audacity can't, or what in your workflow you think is being held up by Audacity. If you need functionality that Audacity can't provide, then sure it's probably worth money to upgrade. You said Audacity is working fine for you, though, so it's not clear what your goal is with upgrading.
Very good points here. If you want to use a computer as the actual recording machine, you might want something more bulletproof/crash resistant. If it's just for post-processing, and you're happy with the feature set, then keep using Audacity until you burn out on it, or run into an actual problem or limitation.
There are *definitely* other options out there, and they don't all cost an arm and a leg, but each has a learning curve, and its own interface and structure. It can be very informative to learn a few different systems, if that suits your style. I've worked in Protools (project version) and Digital Performer, and they each have their strong points. But I do almost everything I can in Audacity. I did get a free copy of Sound Forge with my DR-70D, so rather than sell the code for $50 I figured to use it. It's nice for plugins.