A good performance appraisal system will let you get the most from your people and weed out those who can't or won't do the job. They'll also provide you with some defense if you're sued over a termination. But there's a catch. You have to do things right.
There are two different systems that are described as "performance evaluation." One is the formal system.
Most large companies have a system that specifies when and how and using what forms an evaluation will be done. Companies sell variants of these systems to smaller businesses. This is the formal system and usually calls for an annual or semi-annual evaluation.
What the vendors of these systems don't tell you is that, if the formal evaluation is all you do, you won't gain any of the benefits. In fact, you may be worse off because you think you've got some protection that you really don't have.
A performance evaluation system that brings home the benefits starts with supervision. It's rooted in the ways that direct supervisors, shift managers, team leaders, or whatever you call them, deal with the people who work for them and how they encourage, counsel and correct performance and behavior.
The supervisors need to be trained so that they do certain important things. They must be able to set clear and reasonable expectations and check to make sure they're understood. They must provide frequent and usable feedback on behavior and performance. They must learn to document effectively.
The documentation is critical to having any protection in case discipline, termination or a promotion or raise are challenged. Documentation should be done as close in time to the event as possible. It should deal with behavior or performance, nothing else.
If your supervisors do a good job of supervising, there will be no surprises at formal evaluation time. In research I did on top-performing supervisors, I found that they were more likely than their peers to have forward-focused evaluations and less likely than their peers to have complaints.