I did one. You can only get credit once, no matter how many you do. Make sure you have one of the "supported" cell providers, though (learned the hard way US Cellular is not) because if you don't you will not be able to cancel by texting and have to phone them. If your trial happens to end on a weekend you won't be able to phone them to cancel and then you will be charged. They told me they "might" refund half my money if I was billed (have to wait for my cell bill to come in) and then call them back since I didn't cancel during the term. Frustrating. But my bad for not thoroughly reading the terms and conditions.
If I had had a supported service, I would have been able to text to cancel and it looked very easy. It did credit fast, too.