I just converted a client OFF of Everest to something 20 years OLDER and much easier to use. Frankly, I was stunned to see how difficult Everest was to use and how poorly it was designed.
Examples: Even though Everest maintains 160 columns for each Order Header, "Time Entered" is not one of them. So go ahead and try to schedule a call center with no Time Of Day history. To review a single customer's history (and presumably answer their questions while they're on the phone), you have to open up 6 different windows!
My general suggestion: Write a "Test Drive" based explicitly upon your client's business. Have them enter an order and take it through every stage of it's life (reserve or commit inventory, print, pick, ship, confirm, bill, collect, post, etc.) The test drive should also hit every function needed to do this, setup a customer, SKU, GL Chart of Accounts, etc. Then have the appropriate person working for your client do their job on that system. DO NOT touch the mouse or keyboard. DO NOT let the vendor touch the mouse or keyboard.
If they can't enter, ship, and bill an order in a couple of hours with less than one day's instruction, eliminate the system and save yourself a lot of expense and headaches down the road.
Naturally, most sales people will object to this approach and use any tactic to avoid it. Make it clear to them and your client that this must be the plan. The test drive method is infinitely more effective than the old RFP approach. When the poor systems are clearly failing the test drive, the saleman may object with questions like, "Are you going to train your people or not?" which you'll calmly answer, "Not if it takes until 2012." Good luck.