Working with PI for 15 years I would say more often than not an attorney will do nothing but cost you 33% of your case. There are exceptions to this however. If liability is in question or injuries are extensive, then a good PI attorney is worth his/her weight in gold. They attorney in the end will have a better chance negotiating with doctors and hospitals to lower bills, therefore freeing up more case for the injured party (and themselves).
Just keep in mind, couple of things. PI claims settle low for the most part. Gone are the days that you would see 3x medical bills. Now cases settle in the 1.5x medical bills. Of course nothing is set in stone. Much of the time the doctors notes, accident description and supporting evidence such as x-ray, MRI, CT, computerized ROM testing are critical in demonstrating injury.
Cases with low property damage are also used against the claimant. There is a false perception that property damage equates injury (The insurance companies exploit this misconception to the juries). With the construction of modern vehicles, they are able to absorb a great deal of force without much damage. This is due to the federal bumper impact requirements. This has a negative effect though. The energy that would be absorbed through ride down is transferred to the occupants causing increased injury at lower speeds. The up side is, the vehicles now are constructed to crumple at higher speeds in a controlled predictable manner. This means lives are saved. So in effect it is a trade off between increased lower speed injuries and saving lives at high speeds.
I will add this, if your not really hurt, dont go try to make a case out of nothing. This drives up insurance cost and can lead to fraud allegations. The insurance companies take that serious and may videotape you when you least expect it. If you are hurt, get yourself looked at.