I'm on the grandfathered unlimited plan and also have to deal with throttling some months. I don't think AT&T should throttle at all, because the term "unlimited" shouldn't be applied to a service that "limits" the speeds you get.
If AT&T is going to throttle then I wish it was at a speed of 2mb up and down. That's still compromised, but usable. Probably not gonna happen though, since AT&T wants to punish unlimited users until they switch to a tiered plan--that's the whole point.
The new mobile share plans currently only make sense for families. The individual plans are still cost prohibitive compared to the grandfathered plans. Eventually, I hope the price comes down to the point where I can ditch my old plan but that's not the case now.