I'm afraid I haven't witnessed many IFMIS successes directly myself. John Short and others talk positively about the FreeBalance system set up in Kosovo.
Bill Dorotinsky put some advice on the IMF PFM Blog a while back, including this, which seems sensible:
'FMIS versus IFMIS. Much of the work in automating PFM systems has focused on implementing an integrated financial management information system, including general ledger, accounts payable, accounts receivable, procurement, payroll, asset management, debt management, budgeting, etc. This approach might be too large to implement effectively, in a timely fashion, or to achieve results. It might be better to think of automating some core part of the system, such as general ledger, and accounts payable and receivable, with an eye to adding-on or replacing the system within a few years. Other points in this regard were to start from where you are in terms of PFM system development, rather than from where you want to be, and also to recognize that not everything may need to be automated. As PFM systems evolve, the needs will change, so the scope of the automation can be expanded. Given the rapid change in technology, it may not be feasible to plan all of these potential needs or IT options in advance.'
I attach some useful documents on the subject, including Sara Fyson's paper on what went wrong in Ghana, which is enlightening not just from the perspective of IFMIS, but also on how the donor-consultant-government relationship works.