If you're using the append() method in a loop construct I think that's good choice. If the += syntax is present in a loop it will/could have an impact on the performace of the application. It may even result in points being deducted, but I'm not certain that's the case. The graders seem to nit-pick some of the finer details, so I think a StringBuffer is the way to go.
Also, the JDK states that:
String buffers are safe for use by multiple threads. The methods are synchronized where necessary so that all the operations on any particular instance behave as if they occur in some serial order that is consistent with the order of the method calls made by each of the individual threads involved...
As long as the length of the character sequence contained in the string buffer does not exceed the capacity, it is not necessary to allocate a new internal buffer array. If the internal buffer overflows, it is automatically made larger.
. Thread-safe and no buffer overflow = GOOD CHOICE.
[ May 06, 2003: Message edited by: Christian Garcia ]