Straight talk is a joint deal with Tracphone and Wally-mart. Whose towers they use depends on where you are, around here it's Verizon. I use the $45 plan, when I can I'll get a 6 month card, it's nice not having to load it every month. I've got the Samsung with the keys on front and it slides open for the keyboard, and I am surprised it has run nearly two years pretty much trouble-free.
The screen is having troubles (thrown one too many times) but I can still read it, so I keep using it, because our local store only has the ones with a touch-screen on front. Verizon's pre-paid plan is another $5 a month, and I might switch to them because they have a similar phone to mine.
The boss is on Viero, and looked into company phones for me and the other guy, and even with I think 5 or 6 phones on his plan, it still would have cost me another $15 a month, plus I would have no signal at home.