On January 17, 2012, Google announced that advertisers would be able to see impression share on the adgroup level.
This feature is now available!
Impression share used to only be available on the campaign level. If you wanted to know the impression share on the adgroup level, ... you couldn't. Let's say you wanted to maximize the impression share on a keyword or a particular set of keywords. To measure how frequently your ads are appearing for your given terms, you would have had to create a separate campaign to isolate these important terms just so you can get the impression share metric.
Now, you can simply make an adgroup within a campaign and structure it however you'd like. For example, you could put one exact match keyword into a single adgroup and determine the impression share for that given keyword. Or, you could put all three match types of a keyword in one adgroup and determine impression share that way. The possibilities are endless now.
Working at an agency like I do, a common issue I run into from clients is "I searched for my ad but I don't see it. Why don't I see my ad?" There's always many reasons why clients don't see their ads. One of those reasons is impression share. Now we can determine if ads are not serving as much as they could for particular adgroups. This also helps us search marketers focus our attention on important keywords that aren't triggering ads as frequently as we'd like.
We can now see exact match impression share as well! This metric helps you understand how thorough your keyword list is measured by the exact match impression share.
This is why Google dominates - innovation, they listen to feedback and they have the talent and willingness to make it happen. Wish I could say the same thing for Microsoft Adcenter.