Many SAP projects go over budget. It is a large complicated piece of software - which many do not understand. Like any vendor, the ease of installation is over sold by SAP and confirmed by implementors wanting the work.
The biggest issue that any implementation has is under estimating how much the business process will change.
I worked on a few implementations, the biggest failure was also a success. The system did exactly what the company wanted. However, the company did not understand it's own business and thus asked for the wrong features. Once implemented the company's operations came to a stand still as they could not follow the business process as implemented and was eventually bought by their biggest competitor.
I no-longer work with SAP, but I think it is a great product - just be sure you understand what your business process is and how that process will change once implemented in SAP. Rushing a project and taking shortcuts is a surefire way to end up with a failed project. Oh, and any project that lasts as many years of this, will have had 300% turnover of the implementation staff, so plenty of time will have been lost rolling consultants on/off the project.