It does not work well on Google Chrome as the Choose Flights page does not fully render. It is OK on Internet Explorer 8, Safari,5.1.7 and Firefox 16.0.2 (all on Windows 7).
I like the idea of showing the lowest round trip estimate according to your original selection, but it is a bit confusing when you go to select the return trip. It shows "Included" instead of a price for the return fare - that is for the flights that are within your original selection parameters. It also shows +$nnn where nnn is the additional fare over and above the original quote for additional features or more costly return trips. A negative fare difference will appear if the return trip causes the original quote to drop.
It will be interesting to see how this plays out.