You can now see your ad extensions performance separately within AdWords:
In order to help you manage and track the ad extensions in your campaigns, today we’re introducing a new Ad Extensions tab within your AdWords account. From the Ad Extensions tab, you can see statistics for any of your campaign ad extensions. You can choose to see "All" or "All but deleted" extensions, and you can also choose the type of ad extensions to view, such as location or product extensions. For example, if you select “Locations,” the tab will display all location extensions for that campaign (on the campaign details page) or all campaigns (in the “All Online Campaigns” overview).
Doesn't seem useful in its current form. How about specific info on how many times the ad extensions have been expanded or the expanded products clicked on? This tell me 150/250 clicks were on an ad which included an ad extension. Great....
Agree with Tonearm. This really doesn't do much for me besides show the lift at a macro level to a campaign. Specifically for site links, I'd like to see it broken out by individual site link so you could test different locations for each site link and replace poor performers. I'm sure it's being worked on...right?