Its important to know what you want to do. If you want to run 'apt-get update' on lots of servers, clusterssh is reportedly an easy-to use and effective tool. See Kyle Rankin's article on clusterssh at the Linux Journal.
If your work on different servers will require logic loops, putting, getting or otherwise interacting with each host (or possibly having to interact with a host because of an error condition) a tool such as Fabric (which has just hit its 1.0 release) is invaluable.
You'll probably enjoy most using Fabric if you are a pythonista.
In either case one is probably expecting to do fairly simple tasks on the remote servers affected, and dropping in specially on any problem hosts. However Fabric provides more options for dealing with variation.
One thing that is generally not handled well is dealing with multiple hosts behind a gateway host, requiring one to connect first to the gateway and then port forwarding from there. As ssh itself does this quite simply, and one can set one's
.ssh/config file to something like
LocalForward 9100 192.168.9.11:22
LocalForward 9101 192.168.9.12:22
In order to do the equivalent of
ssh dc1mail 'cmd' &&
ssh dc1phones 'cmd' one has to first bring up
ssh gateway. It would be nice if tools like clusterssh and Fabric would permit this to be done as part of their tools. Naturally, you can do this yourself by hand.