Personally, as far as a *good* F2P model goes, I'd liken it very much to collecting. This especially applies to TF2. You can play it normally and get normal, small rewards in exchange for large investments of time in gameplay that ends up being the meat of the experience, and compliments the reward well, or you can buy what you want immediately for a greater monetary investment and have instant gratification. Heck, you can even take a gamble with crates and keys, if you're that type. With the TF2 trading post and even the ability to sell TF2 items for steam wallet money, it's become a hobby within a hobby, and Gaben's a goddamn genius for making it work so well.
The only thing I can NEVER defend is on-disc DLC and always-online single player. Such models are formed from nothing but pure greed and foolishness.