I have 2 C5 with the Halltech, and I couldn't be happier. (paid for both of them) I haven't had any other system, so I can not honestly offer a comparison. That said, I would buy the Halltech again. Best looking system, and my Z06 picked up 3 MPH and .4 in the quarter. Best time of 12.42 @ 115.97 with a 2.19 60' time. Having participated in a dyno day here in So Cal, it appears that each of the "bottom breather" systems appear to be close in performance gains. My 2K coupe - auto runs 12.9 @ 109 with the Halltech and powerloader.
I am a very happy camper.