I bought my first mac in 1986, and I've used Apple products ever since. I currently own a 13" Macbook Pro, an iPhone 4S, and an iPad 2. I guess that you could call me an iSheep.
iSheep is a derogatory term for someone who has been brainwashed into joining the Apple cult or is so image-conscious (yet sadly out of touch) that they must own "teh shiny." iSheep buy whatever Apple is selling, regardless if they need it or it is any good.
An example of iSheep market power is the adoption of the USB standard by the computer industry. Released in 1996, USB initially struggled to gain traction. Computer makers were reluctant to build USB ports into their computers because there were barely any compatible peripherals. Peripheral makers were reluctant to build USB into their devices because there were barely any computers with USB built-in. Steve Jobs changed all that in 1998 with the introduction of the iMac. The iMac did away with all other legacy ports, forcing users to buy all new peripherals using USB. The iSheep lapped it up... and the USB market boomed.
Another example is iMovie. Apple started bundling iMovie with iMacs in 1999... and home movies with cheesy iMovie effects boomed. Microsoft countered by bundling Windows Movie Maker with Windows XP in 2001, but iSheep continued to bang on about how it was so much easier to make digital movies on a mac. iMovie became synonymous with consumer video editing software in the same way that Kleenex became synonymous with facial tissues. I've never seen data on this, but I'd love to know the percentage of iMac owners who used iMovie compared to the percentage of PC owners that used Movie Maker.
Having iSheep around is handy if you happen to use Apple products. A couple of weeks ago, Chitika released a study stating that 93% of active iPhone users were using iOS 6. This is great because it encourages app developers to take advantage of the latest APIs. The percentage of Android users on Jelly Bean is much lower. This is only partly due to the fact that Apple is responsible for pushing iOS updates to users while carriers and OEMs are responsible for pushing Android updates to most users. Apple has also done any amazing job of training iSheep to push the update button when it pops up. And buy things through iTunes.
When tech writers and analysts discuss the app stores for Windows Phone and BB10, they invariably talk about the twin challenges of convincing app developers to develop for a platform with few users and convincing users to invest in a platform with few apps. But what would happen if, instead of announcing iOS 7 at WWDC a few days ago, Apple had announced an entirely new mobile OS for phones and tablets that would be incompatible with iOS apps? Would developers wait for the users, or would developers pile in, confident that the iSheep will be there? For sheep, that's one powerful market demographic.