In most situations you should rollover any former employer retirement plans, whether it be a 401k, 403b or other type of plan. There are rarely advantages to leaving your plan with your former employer and there may be several disadvantages.
The primary disadvantage is that you are not the customer of the financial institution, your former employer is. You may learn of changes to the plan, your investment options or fees after the fact - too late to make a change. To your former employer you are simply another potential liability.
Also, another issue arises when you reach age 70½ and must take required minimum distributions (RMD's). If you are still in your former employer's 401k plan, you lose the flexibility of where to take your RMD's from.
We would be happy to look at your current situation and help you decide if a 401k rollover is the best course of action.