In terms of numbers, F2P is incredibly solid. 4 to rez, 5 strength is nearly unheard of in sentries. And bouncing any runner card is a potentially devastating effect. And a nice tag to boot. Sure, it won't stop a determined runner, but like a lot of NBN ice, it isn't meant to. It is very taxing, costing 4 to pay through or break with the godtier killer Bukhgalter, and very often more than that.
Mostly, this is a neat toy for Acme. There, this sentry will always be "live", as long as it stays the outermost ice of a server. Of you could park a Data Raven in front of it as the server gets bigger. I expect this to become a staple in said corp, and we might also see it in other yellow decks once in a while.
Also, lovely flavor. A play on the videogame concept 'free to play' (see GameNET, who also gain 2 bucks if you use F2P's ability instead of breaking its subs) and how you can interract with this ice without programs. For a price.