May be if you discribe your situation you will get a more detailed answer.
If your case is simple - just to file the papers, just fill out your I-485 by yourself or use inexpensive paralegal, but if there is some kind of possible complications like crime in the past or arrest, then you need a lawyer who could handle this case.
The only reason i went to the lawyer who was reffered to be by my friend who had a really complicated case not related to mine is because i didn't know if i could file for a GC together with my husband who is out of status and became my husband after i got asylum status. It turned out that my husband couldn't file with me and just filing with a lawyer cost me $$$ though i had all the papers filled out by myself. And after that this super-lawyer did't do nothing!. I had a hard time getting a receipt number from his office to check on my case status and the only time i heard from him was before my interview when he wanted more $$$ to come with me. I refused to use his services and the interview went pretty well without him. My point of view now is if it's something regular without complications - do it your self. If you have any doubts about making mistakes - forums like this are great help. And only if there is something not ordinary - get a lawyer. Good luck.
