What all I can say is take advantage of them and leave when you get what you want.
There are hundreds of Nepali students who are looking for a job and some may never get it because they do not know about these doti companies. I agree they will suck you big time but it is better to get a job and be in Status then going out of status. If you can't find a job on your own, join one of these companies, get some experience and switch your job. If they can use you, you can also kick their ** once you gain some experience by leaving them.
Even though they file your H1 and you are formally their employee, most of them will not pay you until you are placed in a project. This is very frustrating when you don’t get a job for months. Just do not loose your patience and work hard, you will surely get a job. IT market is really HOT these days so take advantage of it.
They will definitely hide many things from you until you join them so ask all sorts of questions. If you had a choice of getting a job in a good company, you would not join dhoti’s company to begin with so it is fair to say we need them as much as they need us. JUST ACT SMARTLY...

Good Luck!