1. Boarding pass is great.
2. Being able to scroll back up to make changes is wonderful. A lot of airline sites are slow, your session breaks when you use the back button, etc. It's nice being able to scroll up, change a date, scroll back down. Well done.
As for the downside, it uses way too much space. This might work on a tablet or touch screen, but it's awful for the desktop. For example, your typical airline uses something like this...
http://i.imgur.com/bEjWWm0.png
A couple of autocomplete boxes, and a calendar picker that pops up. Works fine, it's all visible in a little box, and easy to use. The virgin site stretches this across a handful of pages, or if you're picking further off dates, it might be 10+ pages. That's just bad design.
What they should do is keep a more traditional method of selecting all your airports and dates, then scroll down to the prices when you click continue. I think it would be the best of both worlds.
They also need a method of finding the best price over a given time period. Jetstar is one of my favorites when it comes to this...
http://booknow.jetstar.com/Search.aspx
Pick a flight, say Brisbane to Sydney for a few weeks in June. Click the box to say you want the lowest price. It gives you a nice little graph with all the cheapest flights highlighted, so you can easily choose your days. Actually, the whole Jetstar booking process is decent, and I used it often in Australia.