I use an appliansys cachebox. Works really well - especially for youtube. Things like music videos that subscribers have in their favorite lists, or popular songs dont need to be downloaded through the upstream pipe for every view.
It also brings major parts of websites closer to the user - like reducing the ping to the website for half the content. Many of my customers think that most everyday websites such as the NZ herald (major newspaper) and trademe (our equivalant of ebay) actually are faster and more responsive on their 2mbit rural connection through my network, than at their office on a 20mbit ADSL2 connection with a major ISP.
I also run a dns cache too which helps a bit.
Many people are opposed to caching because of the fear of stale pages - i have no such problem with the appliansys cachebox system. I know its based on squid but whatever witchcraft they do inside the box just seems to work, and works well.
I measure my cachebox performance based on the megabytes it saves me each day and on total traffic, its saving me around about 4 to 8 gigabytes - average number in percentage i see is about 12-15%
For example, when psy gangnam released his new gentleman song, I saw an 11 gigabyte saving that day.
I also remember when lady gaga released her telephone video - in high definition on youtube, i had alot less customers at the time and was trialing the cachebox with a demo unit, it was what made me decide to buy it.
Customers seeing better performing websites was just a side effect.
And again i have never had one complaint of stale web pages being served up.
So I am pro-caching
My upstream is saturated most of the evening - with the cachebox, my customers dont realise - facebook and everything else still appears to run nice and smooth
Attached is an image which is from 1-2 years ago - when I was buying an 8mbit upstream which could burst up to 10mbits - we almost always got the burst avaliable 100% of the time.
Anyhow, you can see what the cachebox was doing in the peak times with the extra throughput on the upstream connection to this router.