Hey, good question. I hope lawyers answer this one.
I think we have to define what "successful" means. Are you talking about money, fame, happiness or something else?
I know a lawyer who still keeps his beliefs and morals. He's doing very well. The problem is he's kinda weird. He has his own sense of morality so I don't really know if he counts.
I personally know lawyers who try not to set aside their beliefs and morals. However, according to them, it's really hard because "kailangan marunong kang makisama." They told me that the problem is the fact that corruption is integrated in the system already. How do they live with themselves? They told me that at least they're trying and they don't really do "immoral" stuff bigtime.
As a law student, I'm hoping that a moral lawyer can still be successful (in terms of money, fame, happines... everything). I really don't think you'd be rich just because you engage in "immoral" stuff and vice-versa. As long as you do a good job then I guess you can be successful. Of couse, what do I know

I'm still not practicing so I'm idealistic. Any lawyers out there who can enlighten us?