My experience has been mixed so far - only 5 years experience with the HR side of an ERP, PeopleSoft. I'm at a large company, and they've had PeopleSoft for over 20 years. The bulk of what we use is customized to the point that there were new things coming out that we couldn't use because they changed some foundational parts of the system to fit with their needs at the time. There was some neat stuff that we were able to build, but our developer team (they handle code/databases/etc., my team is more setup/configuration/requirements/testing) is aging out and they haven't been able to find or train proper replacements in PeopleCode.
We got a new HR Exec a few years ago who wanted us to go through an RFI for a new system. That took about a year, then fizzled, then the HR Exec hired HR Sub-Exec to do the process again but this time include a few other parts of the business to see if we could get multiple areas on one ERP. A year and a half later, we are now about to start the work to implement Workday (HR plus two other major segments). The project is slated to take about a year and a half; HR Sub-Exec said it's "aggressively realistic". There are a number of potential obstacles outside of the project that could end up pushing everything back another 6 months to a year. In total, 4+ years from start to finish. Before we can start though, we need to figure out how we can get 30-some people across departments from the company and 30-some consultants into the same space for an extended period of time.
Workday certainly looked the part in the demos (so many demos), now we just need to see if we can actually set ourselves up to use it properly. I'm going to be the one making all the pretty charts and graphs. HR Sub-Exec has been trying to set our expectations for the implementation work by saying, in a roundabout way, "It's going to suck."
I'm looking forward to learning and adding the experience to my resume so that if my current company doesn't throw money at me during or after implementation (they'll be paying millions for consultants, to randymarsh's point), some other company will as more companies seem to be looking for Workday experience.
To the OP, our overall goal/directive is to make the ERP a good experience for the users, so there's at least one company that's trying to look out for people. Lot's of "Think about the Employee Experience [waives hands and tosses HR glitter in the air]," like they *just* figured out people like it if you make it easier for them to do their job.