My 11.6" air totally replaced my ipad. I sold my ipad and haven't missed it a bit.
For any sort of heavy use needing a lot of storage and/or a lot of cpu power, the 13" is better equipped. I was even shocked that I could run cs5 on my air without it giving me the beach ball of death. Here is a list of the apps I have running on my 11.6" and how they fare to the way they run on my 15" I7 mbp:
1. Microsoft office. I see no difference in the way this app suite runs on either machine. It is bloated and clunky on both machines.
2. cs5, photoshop, illustrator, fireworks, etc.... Runs faster on my mbp obviously, but the difference is not as much as I would have thought. I can have more items open simultaneously on the mbp (both machines have 4gb ram) but the app runs quite smoothly on the air.
3. Firefox, safari. I see no difference in the way these browsers run on either machine. I have multiple tabs open at all times.
4. Skype/ichat. Runs great on the both. I see no difference in these apps on either machine.
5. vmware fusion with xp and a bunch of crappy custom windows apps. This runs better on my mbp. It is slow on the air. I hate having to run it and will be literally dancing in the streets when I can get rid of it on both machines. (I will post video of me dancing in the streets on youtube as soon as this client of mine goes away or enters at least the 20th century and gets rid of these apps.
)
6. itunes. I see no difference on either machine.
7. Final Cut Pro. I can't run this on the air without driving myself crazy. If I want to cut ten second videos, maybe it will work, but for any real video work, the air is not an option.
8. Amazon kindle app. Runs great on both computers. I see no difference.
etc.......
Anyway you get the point. The 11.6" runs most of my apps without compromise.