If you don't have it, I'd recommend Adobe Lightroom and then use Gimp for the occasional 'advanced' edit. Most of the reasons are already outlined in this question. Photoshop is nice, but its not meant to deal with the huge number of photographs you can do from a real shoot. Its a workflow thing.

I find 90%+ of the basic tweaks I need can be done in Lightroom. Its a faster, more efficient workflow. Lightroom is designed for you to make all the small tweaks to your photos fast and efficiently without worrying about changing files around, 'saving' new copies, or changing your mindset for every picture. It saves your changes in metadata and then reconstructs the changes from the metadata instead of saving an altered photo. You 'run' from photo to photo in Lightroom, making the changes quickly or even applying batch changes for whole sets of photos. Its much faster. For the every 'blue moon' edit that I need that Lightroom can't do, you can set the Gimp to be an editor in Lightroom.

It (the Gimp) may lack some of the really advanced features of Photoshop, but in general its pretty suitable. There are plugins for many features (like the content aware fill is provided somewhat by the resynthesizer plugin). We have another question on the differences of Gimp vs Photoshop already. The UI is often a big complaint and can take some getting used to, but after some usage - its acceptable to me.

As Sean points out in a comment above, Adobe Elements is also an option. Its got many of the features of Photoshop, at a fraction of the price.

Piknic and any online editor just isn't in the same class. They're far, far too slow to work with at any real scale.

As far as 'worth it' goes - that's something only you can decide given the benefits and differences above.

At the very least, download a trial of Lightroom and Photoshop. And download an actual copy of Gimp. Start your workflow in Lightroom and see how often you think you need to even go to another editor.