I use the Speedchain, and the increase in clubhead speed is around 10mph. I think this is fairly representative of what the average person might expect from using the chain regularly.
I now use it to help me with lag ... The flow of the chain helps me to sequence the downswing in a way that when I do it right, I get the sensation that my body is part of the chain and flowing with it.
It's easier on the shoulder joint than say swinging a heavy club like Momentus. There is much less centrifugal force acting on the shoulders. You can consider it to be a low impact form of exercising your golfing muscles.
I have a used carpet, 2 meters by 4 meters, and I tie the end of the chain down to a pole using an elastic bungee cord. This prevents the end of the chain from getting too close to your right foot. (ouch!

) I also use a small wooden platform ... about 8 inches high. Swinging the chain whilst standing on a raised platform gets more of the chain in the air and thus you get increased resistence. The chain seems to flow better too.
Anyway, whilst at first glance it seems expensive, it's something that is fun to use and does what it's supposed to. Worth the money, IMO.