Seems like it could be useful for a researcher or something. It screams "vitamin" to me. I'm very reluctant to install a new plugin. It has to be very compelling and this just isn't (for me).
Even if we can improve the search experience on Google just a little, we think it could be huge. And the lowest hanging fruit seems to be the RHS of the results page - there must be better uses for that space, especially with screens getting larger.
Reassuring people that the plugin is lightweight and won't compromise their security is also very important. The history recording might be great, but it sounds creepy.
I don't know if it's true, but a dark themed web site might not be a good idea when trust is important. A/B testing on install ratio?
We've de-emphasized the history part though have kept it in the product since some of our users really love it. In the latest version you can completely disable the recording elements (you could always turn recording on and off).
Interesting thought on the dark background. I hadn't thought that the color might influence trust in that way, but definitely worth testing.
I would default it to off, otherwise, pretty evil.
Selling a Vitamin is a considerably harder sell than pain relief.
Some links for the size of the vitamin market vs. painkiller markets:
The largest vitamin company makes $80 million a year in profit: http://www.foodnavigator.com/Financial-Industry/US-vitamin-s...
The total spent on painkillers in the US is $11 per person, which comes out to about $330 million gross, which probably means less than $80 million total profit: http://www.euromonitor.com/Specialist_products_to_revive_ana...
[1] Except aspirin, which is being taken for its blood-thinning effect rather than its analgesic qualities.