There has been long debates about the performance between Implicit cursors and Explicit cursors. In the past, people(probably influenced by the book Oracle PL/SQL Programming ) believed that Explicit cursor is faster and efficient. However, there are oracle professionals, e.g. Tom Kyte, Tim Hall..etc, now suggest that implicit cursors are more performant than explicit ones. Personally, I opt for Implicit cursors because of its simplicity(remember KISS?) and less error-prone.

Below is some reference links about Implicit and Explicit cursors
http://www.bhatipoglu.com/entry/27/implicit-vs-explicit-cursors-performance-analysis
http://www.dba-oracle.com/plsql/t_plsql_cursors.htm
http://oracle-gurus.co.uk/ExplicitVsImplicitCursors.aspx
http://sql-plsql.blogspot.com/2007/05/oracle-plsql-implicit-cursors.html
http://radiofreetooting.blogspot.com/2007/01/explicit-cursors-rip-not-quite.html
http://awads.net/wp/2007/06/21/cursors-again/

Tom Kyte's explain on why implicit cursor is faster than explicit one.
http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:1205168148688
"PLSQL, like java, is interpreted. We have some C code that is a PLSQL VM (virtual
machine) that runs PLSQL. The more PLSQL code you write, the more we have to intrepret.
Intrepeted code runs slower then native code (object/binary code) does."