Gygaxian Building Block since the codification of the Feat and the Template in 3E. I'm all for allowing magic-users to deal with the devil in exchange for more raw power, but I think the patron rules in the beta have two flaws.
1. Building a patron from scratch looks like hard ass work. For the full monty treatment you need an Invoke Patron results chart, 2 or 3 new patron-themed spells (in a game where designing new spells is non-trivial), a Patron Taint chart and a Patron Spellburn chart. Putting that all into spreadsheet fields, that amounts to almost eighty individual blanks you need to fill. At that point what should be wicked fun starts to look like homework. The prospect of starting on my first custom patron gives me the same feeling I got from trying to make high-level NPCs from scratch in D&D 3.5. That's not a good thing.
2. Even though I don't want to add one more item to my checklist of crap need to make a new patron, the patron write-ups should specify the an Agenda for each patron. From a DMs point of view there's no juice in making a deal with the devil unless the devil is in it for something. I've found that coming up with ideas for new patrons is a snap, especially when I've written a crapload of demon-themed custom spells. But unless the patron has something concrete they are trying to achieve in the campaign world there's just no point in putting him/her/it into play.