I've been SIXT Platinum since 2002 and I must admit, it's more a name than a "real" perk.
The Fast Lane is MUCH slower than that of Hertz Club Gold. I have yet to just walk up to my car and drive of, I've always had to goto some sort of desk, maybe I've just rented at "off" locations, but that's annoying. I've never received an upgrade yet (then again, I always book the best possible car wherever I am at). The discounts (WUT) oh please, haven't really felt any discounts from SIXT, then again, SIXT are one of the most price competitive imo. You get better discounts with Hertz, specially with airline programs.
One of the best reasons I use SIXT, isn't the tiers, it's their cars! Their cars are ACE compared to ANY other rental company imo; specially here in Europe. They use top of the line German makes; I like! So use SIXT for their cars, not their card tiers, waste of time. This is all my personal opinion.