For the sites I operate this is my 'general health' indicator, bandwidth says a lot more than my alarms, if there is a problem it usually shows up in the bandwidth graphs before the alarms trigger (unless it is a power failure, but those are extremely rare).

Our providers make them available to us, and this has been the case with any provider that we've had to date (the planet, vxs, leaseweb and a couple of smaller ones), I'd imagine amazon has them too.

According to the Amazon FAQ you have to use 'cloudwatch' to get at this data:

"An Amazon VPC router enables Amazon EC2 instances within subnets to communicate with Amazon EC2 instances in other subnets within the same VPC. They also enable subnets and VPN gateways to communicate with each other. You can create and delete subnets attached to your router. Network usage data is not available from the router; however, you can obtain network usage statistics from your instances using Amazon CloudWatch."

You may have to do some arithmetic to see if a link got overloaded, one telltale on the bandwidth graphs is 'flat caps', where in spite of the machines inbound limit still not being reached you see a fairly flat top on the in or outbound bandwidth graph on several machines at the same time (if they're on the same segment, which on amazons infrastructure could be quite hard to figure out).