I searched a few strings for Hopper.com, and I wasn't able to find any existing topics or posts on it.

The way I see it, Hopper.com is divided into three main "products," at least from the consumer-side:

Deals
View good fare deals by origin city, similar to TheFlightDeal wrapped in a pretty UI.

Flight Explorer
Browse a longer list of low-ish fares from an origin destination, filter by timeframe and length of stay. Similar to Airfare Watchdog.

Reports
This is by far the most exciting part about Hopper, in my opinion, and also the product featured on the site's landing page. In short, "Reports" puts an incredible amount of detailed and automatically-generated fare data at your fingertips. I imagine it's data like this that drives the "Buy Confidence %" metrics on Kayak.

Why Hopper took so long to "launch"

Hopper | Crunchbase

I still prefer feeding RSS-enabled sources (e.g., The Flight Deal, FT Mileage Run Deals) through IFTTT to push notifications to my email in (or close to) real-time. This also offers more opportunities for targeted results based on search strings.

But for more casual travelers, I suspect Hopper's comprehensive suite of products and UI-friendly design will give existing flight discovery sites a run for their money. It's ironic, because I'm pretty sure Hopper relies on these RSS feeds to gather data - at least to some degree.

edit

I originally wrote this post for FlyerTalk, but I've since been cross-posting elsewhere on the Internets. Hope that's ok. If you're interested, you can read my own review of Hopper here.