It does link to Geocaching app in that it tracks finds. With the c:geo app I can find any difficulty or terrain. I use both apps when I am out looking though. It is a stand alone app that is free. I am just a cheap person I guess.. :)
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Unless I'm mistaken, I believe the c:geo app only runs off of your existing geocaching.com account, and has the same limitations unless you upgrade your account.
The upgrade isn't necessary to hunt for basic caches, but it will unlock "premium" caches. At only $30 a year, I pay for the upgrade. It's well worth it to me for the additional features.