The soft limit on Straight Talk's unlimited data was highlighted by Mashable, though the issue has been around online for longer. I found a thread on an appropriately named site called "Pissed Consumer" from January of 2012 where numerous users complain that they were misled by Straight Talk into thinking their accounts were truly unlimited, only (for some) to be completely shut down after downloading too much data. Though a few users in the thread claim to have downloaded gigs upon gigs of media via their Straight Talk plans, plenty of others complain that they weren't told about the unstated limit of 2GB per month until they called to ask about their accounts.
This doesn't necessarily mean you shouldn't use Straight Talk, however. A 2GB limit is a relatively standard tier when it comes to smartphone data, so if you can live with such a limit (and slow speeds after that), $45 per month is still a decent deal compared to the other US options. The main question is about how the company is marketing its product, and what subscribers are led to believe when they sign up—remember, sometimes you get what you pay for.
Straight Talk and Walmart did not respond to our requests for comment by publication time.