as far as i can tell the 2007 traces energized ckts., the 2008 traces de-energized ckts., and the 2011 does both. I've been using the 2007 for about 4 yrs. now and it works great.
What sets the 2007 apart from the $30 generic variety is that it generates 3 harmonic signals compared to 1 for the cheaper version making it a lot more reliable.
What is the difference in price between the 2007 and 2011?