Galera cluster latency issues

I ran into a seemingly well written article today on setting up a Galera cluster with master-master replication on Ubuntu. The questions that appear unanswered to me

a. Suppose I setup a cluster with 5 servers scattered across the four corners of the globe. Will the efforts at keeping them in synch result in a significant latency issue that will be to the detriment of the system as a whole?
b. Am I write in thinking that provided I get Galera up and running properly I can pretty much leave my PHP PDO code unchanged in the blissful assumption that Galera is handling replication so well that I have a cast iron guarantee of being able to access the same data on each server?

So as Galera is called “virtually synchronous replication” you may expect that the latency between the nodes will affect the whole cluster performance. It also applies (however in a bit different extent) for the individual nodes capabilities as the slowest node may again affect whole cluster operations.
Regarding accessing same data on each node (server) - see the causal reads setting if you really need it.
From my experience you will want rather reliable WAN links between the nodes.
Also since Galera 3 there are improvements to handle WAN scenarios better:
http://www.mysqlperformanceblog.com/2013/12/19/automatic-replication-relaying-galera-3/

Some more hopefully interesting lectures for you:
http://www.mysqlperformanceblog.com/2012/01/19/percona-xtradb-cluster-feature-2-multi-master-replication/
http://www.mysqlperformanceblog.com/2013/03/03/investigating-replication-latency-in-percona-xtradb-cluster/
http://www.mysqlperformanceblog.com/2012/06/14/comparing-percona-xtradb-cluster-with-semi-sync-replication-cross-wan/