I, personally, had a mixed experience. Since, I did not need to apply for H1, I was ok on that part. Talking about consultancy itself, as rustymood said, they are not your friend. They fail in delivering their promise about on site job help. Once you get a job, they don't care about you and do not help you in any way to sustain your job.
I completed one project as a Business Analyst and I had a very bad experience. When you need help and call consultancy, they don't care about you and rather blame you for not performing. I got a very good job with decent pay and I had to endure lots of stress during the first six months of the project. After six months, my project was over and had to come back to Boston.
Even though, I had a bad experience, I would suggest others to give it a try, if they want to make good money and don't care about the job itself. You will make pretty good dough but the first six months of the project will be hell and be prepared for it.

Thanks,
RM