If it's free it won't cost you anything. I have taken up that Virgin offer and love my phone. £15 a month including calls, texts and unlimited internet.
it's a phone that Virgin appear to have a lot of stock of.
They were trying to get me to take one a while back, but (for various reasons) I need a Windows Mobile phone. Rather annoyingly, i have since found out that the phone I told them I wanted was one they stocked, but is now out of stock. Grrrrr.