All service providers (IBM or third-party) will ask you to sign a maintenance agreement with them. A maintenance agreement is a form of insurance, but is only necessary after IBM's warranty has expired. During the warranty period, you should always call IBM for repairs, no matter who sold you the system.
Before the warranty is over, you should investigate several service providers (including IBM) and compare them. If you think you will enjoy peace of mind by having IBM's service, go for it. But always compare the following:
Professionalism. How are you greeted on the phone when you call? If the other person is not available at the moment, do they call you back when they promised? Do they treat you like a VIP or like a burden?
Quick turnaround. How long after you call does someone show up to fix the computer? Can they guarantee 1-hour turnaround?
Quality of service. Does the technician have the proper tools to perform the job, or is he using the same Phillips-head screwdriver for everything? Does the technician treat the machines with care and avoid jarring and violent movements?
Knowledgeability. Does the technician have access to the manuals required to perform the job? Has the technician been trained properly, perhaps by IBM?
Price. Price should always be the last consideration because it is the least important. If your computer goes down for a long period because the technician cannot fix it, it will not matter that you saved a substantial sum of money in monthly charges. Your firm will lose revenues, and you might lose your job.