Based on this data, you can have really fast responses around the world if you provide your services in all regions.
However...
It is really difficult to get their cloud services to work across Regions (AMIs not shared, security not shared, RDS, Cloud-formation only works in one region, etc.). If you want to run in multiple regions, you need to use EIBs for your database servers (mysql, mongo, etc.), setup your own replication and fail over, etc.
All in all, though, a great service (though at first, I didn't like the entire degraded server problems but realized it has forced the issue of not assuming servers will not fail).
I would point out that global replication is one of the things that makes CouchDB's master-master replication so tasty.
You can't accomplish the same thing with Mongo in a R/W environment since you can only write to the master in a replica set. You could have multiple read-only slaves in other geographical locations, but that isn't always helpful.
You can also somewhat fake it with a much more complex mongo-replica-set-per-region sharding configuration, but then you don't have the same data available in each region in each replica set.
PostgreSQL in 9.1 added master/master streaming replication which is nice. Not sure what MySQL does in these situations.
This is one of the biggest failings of SimpleDB that I think keeps it held back from being a huge success, the region dependency of it and the seeming lack of support from Amazon (the only product whose forums frequently go a week without a reply).
Had SimpleDB has an optional global redundancy for quick read/writes in all locations, I think it would have been the killer product a lot of people were looking for.
Instead I have to roll my own with a fully connected CouchDB graph of servers. Ahh well.
Based on this data, you can have really fast responses around the world if you provide your services in all regions.
However...
It is really difficult to get their cloud services to work across Regions (AMIs not shared, security not shared, RDS, Cloud-formation only works in one region, etc.). If you want to run in multiple regions, you need to use EIBs for your database servers (mysql, mongo, etc.), setup your own replication and fail over, etc.
All in all, though, a great service (though at first, I didn't like the entire degraded server problems but realized it has forced the issue of not assuming servers will not fail).