PC Manufacter’s Self-Support Offerings

Does anyone have any experience with either Dell’s, IBM’s, or HP’s enterprise support options? We currently have Dell as our preferred computer vendor but our contract is almost up. We are sorting through the responses to our RFP and want to compare the support plans that the three companies offer. Our technicians are Dell certified at this time and they service the systems themselves and are reimbursed for each repair. Parts are shipped overnight as needed. Overall we are happy with this arrangement and would like to have the same program if we choose another vendor. Does anyone have any experience with these vendors and their self-support programs? Your response is greatly appreciated! Thank you.