Yes, they do throttle your data speed if they think you use over a certain amount of data..... (they won't tell the users exactly what the data downlad useage limit is, for their "unlimited" data plan) - the word around the internet is that Straight Talk throttles you if you use over 2 gig of data in one 30 day period.
they slow your data speed down to next to nothing (throttle) and then you call and complain and they will reset something on their end and your data speed will be decent for one or two days and then back to no speed at all so you call again and go through the same process .... the reps I spoke to are nice enough but apparently they really have no control over really fixing your data speed, all they can do is to do a temporary fix that lasts a day or two.
from what I gather after reading other forums, your data speed really won't go back to normal until your next billing cycle.