Most of the people providing responses don't know what they're talking about.
Straight Talk started using Verizon 4G LTE a long time ago, utilizing Verizon 4G LTE prepaid tower infrastructure, and you use the Verizon compatible ST SIM to keep it on Verizon towers.
I just got done with an almost 4,000 mile road trip using my originally Verizon - purchased Samsung Galaxy S3 with the CDMA SIM provided by Straight Talk, although porting my old number made it slightly more difficult than a direct conversion with new number.
I still run on Verizon 4G LTE speeds, and did so in many states during my road trip.
My little brother is with AT&T and had NO service in many of the places my Verizon CDMA coverage through Straight Talk was working perfectly... 4 bars against the AT&T GSM 0-1 bar unreliability.
My recommendation is to make absolutely sure your specific model is or can be unlocked and be compatible with ST before attempting it. I was told repeatedly that my S3 would not work, and it's worked just great. I've posted and received SMS messages great, including lots of photos and videos, just as I always had before with Verizon, at about half the price, with all of the Verizon 4G LTE functionality I had before.
Don't blindly take what others might be providing in the way of false or ill-informed responses and contact Straight Talk with your specific ESN or IMEI (which has to be clean). If all checks out well, go for it.
I even bought another used Samsung S3 on eBay that is a Verizon specific model (to make sure I remained on the more reliable Verizon towers), that came with the VZW ST SIM, and I had it activated within 5 minutes of receiving it. It's worked perfectly, too, on the Verizon 4G LTE network. No problems whatsoever, calls, texts, SMS, photos, videos, etc.