As graphic designer with a big consideration towards typography I looked to use the best quality web-font service for my own website.
I already used most web-font services in my work. There’s no standard about the use and pricing of those services. Some host themselves the fonts, some ask you to host them, some even ask you to protect the fonts in your own server. Some ask you to pay once and for all, some others ask to pay per page-views/month. And each services, of course has different typefaces, so sometimes if you want an exact combination of fonts you have to deal with different and complex pay methods.
I was looking for quality but most of web-fonts doesn’t have a proper on-screen display because they don’t have good enough hintting, because it’s a work of hell for a type designer. But Hoefler & Frere-Jones did it, and they recently launch their web-font service past July. Yes, they have their own pay method, different to the rest, and being fair a bit expensive, but quality pays off. You pay an annual subscription for using the service and then you also pay for the fonts you are going to use. As the first 5 font packages are for free, I was able to afford the service.
They make a big effort building the “ScreenSmart” version of their text fonts with a great hinting for 9 to 18 px sizes. Which I’m not using as I’m not big fan of small sizes on web (are you paying for that infinite scroll? do you want people get blind?).  So I’m using the normal web-font versions of Mercury (for tittles and the left menu) and Whitney (for the rest), and they look amazing, good quality print fonts with great on-screen display.
Update: after a lovely 6 years of Mercury and Whitney, I moved to IBM Plex, since it’s an Open Source which I can manipulate and play with for my own purposes.