I've got a little bit of history with programming, having gotten my start with Visual Basic. I let it slide as a hobby from a little after VB6 up until just a year or so ago when I got on the iPhone bandwagon, when I decided to take up Objective-C programming.
I now have what I feel to be an intermediate level of knowledge on the language, but I can't help but feel that there's a substantial gap in where my knowledge is, given that there seems to be a substantial bit of the language I don't feel I understand.
Would I benefit from reviewing the underlying C structure, or would I be better served from continuing to practice solely in Objective-C whilst reviewing the documentation for various features?
Edit: Though I've accepted the first answer given, more insight is always appreciated from those with supplementary or opposing views.