From our point of view, the real value of micro instances is architectural. For tasks where the load is small but important (queue servers, memcache instances, monitoring), where before we might have had one small instance and a potential point of failure, we can now afford to have two micros, simultaneously providing redundancy and halving the price (micro = 1/4 small).
This is especially important in AWS, as the dirty secret of small instances is that they occasionally vanish without warning -- in our case this has happened 4 times in 18 months.
Another nice advantage to Micros is the ability to scale up in more incremental chunks. The linked benchmark works well if you use the full capacity of the medium at all times -- but if you need a little less than that, running a cluster of Micros gives you eight smaller stepping stones to find the sweet spot.
When they vanish without warning, do they take the EBS with them?
Is having your OS on an EBS (as is required for micro) a dangerous thing, because the instance vanishing without warning might result in a corrupted EBS drive, making restoration more complicated than spinning up a new instance?
EBS is pretty solid in my experience; I've never lost an EBS volume (on the contrary, I seem to have stacks of them lying around from boxes I killed without killing the EBS... need to do some housekeeping...).
I've had the same experience with smalls as the parent poster. No EBS loss though. The larger instances do seem more stable. No quantitative data to back that up though.
I don't know if 75+ instances over 18 months is large enough to count as quantitative data, but we've never lost a large instance (they've had temporary failures). All four of the permanent vanishing-acts were smalls.
This is especially important in AWS, as the dirty secret of small instances is that they occasionally vanish without warning -- in our case this has happened 4 times in 18 months.