Ok, fired up MobMeter and got some readings:
Without the external cooler running, I'm at 49C Idle and 62-64C gaming.
With the cooler on the low speed, I'm idle at 46-47C and gaming peaks at 60C. The noise is noticeably less than the system fans running on high.
With the cooler on high speed, I'm idle at 44-46C and gaming peaks at 58C. The noise is a little less and lower pitch than the laptop fans on full.
So, I'm getting about a 2-6C drop in temps during gaming and a little less at idle. Looking at other peoples post, my systems seems to run alittle hot to begin with, not sure why.
As for fitting, it actually doesn't go under the laptop. It sits at the back of the laptop. The fans will stick out and draw in the cool air. The laptop sits slightly on top of it and the fans blow the cool air out under the laptop. On high, my right hand gets a little cold from the air constantly blowing on it as I use the mouse. I'll try to post pictures when I get home.
The only down side is when gaming, the CPU fan kicks in at times and you end up recirculating warm air, which likely decreases the effectiveness, though I still see improvement over the stock cooling.
Now I just need to try OC'ing my system and see what happens