I can give a litle insight on this.
I just graduated from Drexel University (Philly), which has a co-op program that SIG constantly employs out of. Even for the co-op program, the interview questions are mostly centered around probability. This is a good microcosm of the type of people the firm hopes to employ. They don't necessarily care if you majored in engineering or liberal arts...if you're smart, you're smart (obviously you need to have showed some type of interest in finance). For the co-op program, most of students are given operations roles, but for those that excel they basically become traders when they graduate. I have a friend, for instance, who's extremely smart and was promoted right away to an assistant trader role and after he graduates will become a full-time trader...pretty good for a 22 year old.
Like people said in this thread, the firm itself is great. They have an annual poker tournament where the winner gets offered an assistant trader job at the firm. In fact, they're obsessed with poker. The president came to my school to lecture about what a trader does and it was basically one big poker lesson. It's not surprising that some people disliked working there, though. You really need to have a quant mindset in my humble opinion. As an example, a lot of people thought I would be a great fit there. I graduated in the top 3% of my college class and played poker over the past few years to pay my way through college, and everyone I know told me SIG, SIG, SIG, SIG. But the culture there just wasn't for me.
So yeah, great company, but make sure it's what you want to be doing. I myself will be applying for the equity research division, though, in a few years from now. I'm pretty sure they don't hire right out of college for this, however.
GL