Never had a problem with SMS. Had Straight Talk on my iPhone 4 for a year before I got the 5. Had to do the sim swap trick to get MMS working where you put a T-Mobile sim in to get the MMS settings to pop up and you manually enter the settings for Straight Talk. Data and imessage work without doing the sim swap. Everything has worked flawlessly since I've had it.
If you go on YouTube and search it up, several people are saying they have issues with SMS/MMS. But the data works flawlessly.
How about 4G? Is is working with ST?