A friend and I both went through this interview process together, although he was applying for a Support Engineer position.
As mentioned before, the interview consists of:
1.) Phone Interview
2.) At home SQL test
3.) Onsite SQL & Finance Test
4.) Onsite Panel Interview
5.) Fit Interview
All of the tests were extremely fair and did a good job of testing your knowledge, with the exception of the Finance test. The finance test was designed so even people with little to no finance background can pass it. They just want to be sure you can learn and understand new finance concepts so you can be successful in the future.
The dress for the interviews is casual - business casual, and everyone I met was extremely knowledgeable and friendly. My recommendation is if you are honest, transparent, and friendly you should do well.
The fit interview was a very foreign concept for me. Going into it, my recruiter had explained this to me as "just a formality" and that it's there to make sure you can be friends and drink with your new potential co-workers. He recommended not worrying too much, having a couple beers, and having fun with your potential new co-workers. My advice is to NOT go in with this same mindset. This is very much still an interview, and although it is in a much more casual environment do not hold yourself to the same standard as the current employees.
My particular experience was roughly 2 hours long with a total of about 20 people.
During the interview multiple people were venting about some professional problems they have experienced on a job (which let's be honest we've all done at a happy hour with co-workers). My particular problem was I decided to join in on the conversation and contribute. This ultimately was part of my downfall, as you should never speak negatively about any aspect of your professional life during an interview.
Additionally there were multiple instances where two people were asking a question at the same time. Under normal group conversations this may not be a problem, but in an interview environment this led to people feeling like I had dodged questions or ignored them.
I gave my friend this same advice and he ended up doing extremely well on his fit interview and got the position (plus I'm sure it helps that he's an awesome guy to begin with).
At the end of the day this interview process left me feeling like there was a missed opportunity for both BMS and myself, as I am very confident I would have been successful within the team and organization as a whole. My hope this that, by following my advice, the same won't happen to anyone reading this.