I will leave others to discuss their experience with radius load balancing. I will instead offer some tips. The best command to see the effect of radius load balancing is "show aaa authentication-server radius statistics".
"In general, the controller keeps a running average of response times for each server. If response times are relatively similar, the load will be evenly distributed. A server with a quicker average response will get more requests (based on moving average response times). If a server has a lengthy delay on one auth, that may skew its being used again, so if a server has not been used for a period of 5 minutes, it’s moving average gets reset to the default so it will get back into the round robin. The server used for a given client will be “sticky” whenever possible to facilitate shorter reauthentications instead of full on auths."
You should use the "clear aaa authentication-server radius statistics" to reset your statistics to get the latest information.
Radius Server load balancing only works for radius and LDAP. 6.4.3.x introduces radius load balancing support for server groups used for Airgroup authentication. Previous to that, only the first radius server in a server group is used for airgroup authentication.