Is it okay to use StringBuilder like this?

StringUtil.java

public class StringUtil
{
  // Pad zeros.
  // NOTE: Assumes base-10 representation of number.
  public static String padNumber(double value, int len)
  {
    // Supports signed numbers.
    boolean isNegative = value<0; 

    if (isNegative) {
      value = Math.abs(value);
    }

    // At least this many.
    int valueDigits = (int) Math.log10(value) + 1;

    StringBuilder sb = new StringBuilder();

    if (isNegative) {
      sb.append('-');
    }

    sb.append( Double.toString(
      // Pad zeros, at least as much as value.
      value + Math.pow(10, 
        // Pad at least as much as length of value.
        Math.max(len, valueDigits) )
    ).substring(1) );

    return sb.toString();
  } 
}