Great little product if you are going to be over clocking your Pi or using it to run CPU intensive tasks. The Raspberry Pi website says that you don't strictly need a heatsink due to the type of chip they use but I've run a few processor intensive tasks and things got a bit toasty so I've put one of these on anyway just in case.
Expensive for what it is and on a pi3 I'm not sure how effective it is.
As a test I loaded stress - sudo apt-get stress. Then use the following line: while true; do vcgencmd measure_clock arm; vcgencmd measure_temp; sleep 10; done& ./cpuburn-a53
This will show you the temperatures whilst running a pretty harsh stress test.
Running this without a heatsink I got up to 80 degrees pretty quickly - probably by the 3rd temp report. I aborted this around 84 degrees and perhaps this would have gone higher .. not sure.
I then fitted the HS and ran the test. This was a slower climb .. perhaps 5 reports to get to 80 and above - it appeared to settle around 83 degrees but I stopped it there.
So the HS isn't going to stop it getting to the magic 80 degrees where the throttling will take place. I think a fan may be the only answer but even then.
Now when considering if this is worth it or not one has to consider real world applications and you'd need to do a watch for temperatures for the application that you intend to run and see if it will hold the temp you require.
Bear in mind that the stress test I've used is probably going to be pretty extreme and it's unlikely that your required app will push it this far.
Given that the pi3 runs much hotter than its predecessors (eg. the pi2 under the same stress test without a HS would get to 65 max) I believe it's better to put one of these on anyway and depending on your desired application consider a fan if it's not enough,
It's very easy to fit to the Broadcom System Chip and it reduces the reported core temperature by 5 to 10 degrees.
The only slight drawback is that some of the GPIO header boards that sit over the Raspberry Pi board don't quite clear the top of the heatsink! I've got around this by using an Adafruit 2x13 Stacking Header, which gives plenty of clearance.