The instructions are simple, install EPEL, then install the VDT repo:
rpm -Uvh http://vdt.cs.wisc.edu/repos/3.0/el5/development/x86_64//vdt-release-3.0-2.noarch.rpm
Install the osg-client (expect a lot of dependencies). My latest build downloaded 140MB of dependencies.
yum install --enablerepo=vdt-testing --nogpgcheck osg-client
Then, go and do your normal workflows. Create proxies, run globus-job-run's, start condor and run jobs. Whatever, just test. If you have any issues, email firstname.lastname@example.org. Or join me on the OSG jabber, email@example.com.
All current bugs (I'm happy with how few there are) with the osg-client can be found on Jira.
Go forth and test...