Just completed our first trip out with new TST507 and love the piece of mind it provides.. But, you will NEVER hear me say that you have to have one before leaving home because it is simply not true. You are buying piece of mind, like you did when you bought those fancy tires (me too, FYI).
I had purchased the TST501 "on sale" last fall from an online source (irrelevant). I installed it and couldn't get one of the sensors to work due to dead battery out of box. When I worked with TST to get replacement and ask about warranty on a less-than-3-month sensor, she asked for the date code on sensor. Turns out sensor was built in 2013 (5 years old when i got them 'new' on sale). After discussing nicely with a lovely support person at TST, I exchanged my "not-so-new" TST501 for a new TST507 model. I used the 501 for one trip, and have now used the 507 on one trip. In my opinion the 507 is a much superior product for many reasons (one being I couldn't read the 501 screen without removing my sunglasses).
Customer support on the TST was phenomenal and efficient and I promised to say that as often as possible so thanks for letting me. LOL