For people that are just 'starting out' and also people that just want to keep their hand in, it's really a good idea to practice writing small pieces of intricate code like tokenizers, string processing etc., rather than just selecting the closest library to hand.
Obviously, if you're writing code commercially, you may want either complete control over some code (write it), or you may want it now (use a lib), so writing all your own code may be an impossible luxury.
Here's a small example snippet I just did. I've been doing software since 1988, but often just gluing (MFC) api's together, and not doing stuff that really benefits me as much as it might. Anyways, here's a gist that implements a 'good enough to use' version of an iOS 4.0-only api as a category on NSData, allowing string searching of arbitrary data:
