There are a number of page monitoring options available (I use the Distill add-on for Firefox), but I'm unaware of any that check for visual changes at the pixel level via screenshot, so this is an interesting idea.
On the other hand, some might prefer to check only a specific portion of a page for changes in a piece of text, for example, and whole-page change alerts might be too general. Additionally, checking twice daily might not be frequent enough for some. I guess you'll get feedback about this and considering making changes if they're suggested often enough.
There are a few errors in your copy that need fixing:
* We monitor the pages multiples times per day. If anyone [any one] of them changes, we'll know. And so will you.
* You will be instantly notified when a page change [changes].
* Product managers can use Sentinel to check their competitor [competitors'] product and marketing sites, such as the homepage and pricing pages.
* Marketers can be instantly warned when competiting [competing] products update their marketing message, allowing them to react and take appropriate action.
* I was super pissed, I cried for a couple of weeks and I decided to create a [tool?] that did the same things, hopefully better.