I've been using this gv only setup for 2+ months and I've never had any extra charges for incoming texts. People reporting that most likely have a setup issue. GV texting is done through the internet and is only linked to your data plan.
To ensure that I had a GV only setup, I made sure to add SMS and MMS blocks to my TMo account. I told all of my friends to use the GV number to call and txt me. If I didn't put a block on txt msgs from TMo, there would have been nothing stopping people from txting my actual cell number and I def would have been charged.