I've been using the string trellis method to grow my tomatoes vertically for 3 years now. And with much success. One problem I've faced every year is the twine breaking, and my tomato plants falling down when they get laden down with heavy tomatoes. Usually string breakage hasn't been a problem for my cherry tomatoes, but it's definately been a problem for every other variety.
The first year, I only used a single strand of twine. The second and third years, I doubled the twine, with no better success. Twine just doesn't last very long when exposed to the elements.
So, this year, I'm using some small yellow nylon rope that I picked up at Home Depot. I suspect it will NOT snap like the twine does!
I planted my tomatoes on Saturday. I see a low of 36 in the forecast for Wednesday night, so I'll have to keep an eye on that!
Here's the tomato garden!