How does the battery life of the new MacBook pro stack up against its predecessor in practice ?

I see a lot of last gen retina machines selling for peanuts now as users rush to upgrade.

The new 15" MacPro's are thinner and boast up to 8 hours of battery life according to the Apple specs.

Didn't the last gen 15" MacBook Pro's also come with 8 hours of battery life on spec ? I want to know how much difference there is in real life usage, since the running time given for the batteries seems to be near identical. The performance difference between Haswell and last gen procs would be hard to notice as I understand.