I went on a J1 last summer with USIT - they were fine - I had my embassey interview about 3 weeks after booking, they had all my info online so I could track it myself, their New York office (one woman in a tiny room in a hostel) was helpful when you get really stuck over there. And I've heard alot of bad stories about Sayit and Go4Less (Sayit in particular), although there are some about USIT too (I haven't heard as many, but I'm sure some people have had bad experiences). If I was going again, I'd probabily use USIT. The sponsor organisation they use (CIEE) is very helpful too.
The J1Jobs site is useless. They had the same jobs online all last year, nobody ever relpies to applications and it looks like they have the same ones online this year too.
If you're looking to pre-arrange a job, start googling resorts and lodges, hotels, amusement parks, all the touristy places and ask about applying for a seasonal job for the summer. Include a short cover letter and resume (one page CV, keep it brief, concentrate on work experience) and maybe a reference. A lot of places wont accept e-mail attachments so include them in the body of the e-mail.
If you need any more advice, just ask.