I'm on Fanfiction.net as MistressMarionette, but I've only written one Bartimaeus Trilogy thing, a sort of drabble. And it was written before I read Ptolemy's Gate. Looking back, my Ptolemy was totally off, and people have told me so (but nicely. They weren't like, "YOU SUCK, BIZATCH! NAHHHHH!" They just mentioned that there were some rather significant differences. A couple of them said I wasn't too far off the mark, but I really was...). I'm working on another with Ptolemy and Bartimaeus again, this time using Ptolemy's Gate as a source. If you wanna check the other story out, it's called Not In a Million Years (Bowles has read it already. And left a very super-helpful review. :D )
And also, I know somebody mentioned this earlier, but Contrarion is a major kick-buttocks author. As is Bowles. They both seriously rule the Bart-fanfiction scene in my opinion...
And I agree, it's hard to find quality on both Fanfiction.net and Fictionpress. I gave up on Fictionpress a long time ago (partly because their search engine thing really sucks. I find it funny that they only include the most LEAST recent stories in their search results.) It's almost a good thing the Bartimaeus section at FF.net is so small, it's easier to weed out the good from the bad from the just plain ugly...
"Hey, we've all got problems, chum. I'm overly talkative. You look like a field of buttercups in a suit."