I switched from Verizon and the process was painless.
However, there is one thing to note, and that is that you're going to need your Verizon info: account number and "billing PIN" - this is different from your account password. Verizon was able to reset my billing PIN via online support chat. After that, once my Fi SIM came in, I popped it in, went through the porting process, entered my Verizon info, and my number was ported. I got a message saying that it could take up to 3 business days to port the number, but it took about 2 minutes, and I quickly confirmed by sending myself a text from another phone.
The switching process is as easy as it can be. I've been loving my Fi service thus far (1 day left on my first month and I'm only at 0.49GB used). Can't complain at all. The app makes it very easy to watch your data usage, and as of the last update, they have a widget that shows your stats.