I'm having a reasonably good experience with a cost-plus. We're just wrapping up. I got bids from three builders, the fee was the same for all of them, so just make sure your builders fee is the going rate for you area.
None of the builders would agree to a holdback of any sort. It's just not customary in this area.
Make sure you trust the builder, if you can't do that, you got the wrong guy for a cost plus. You're putting your checkbook in his hands.
Some one pointed out to make sure your builder has done cost-plus before. Good point! Mine had not. I got him to knock 3% off of his fee since I took on the risk, but we went over by about 5% (plus overages for upgrades), so I came out a little behind. Since my guy normally did fixed price, I was confident he knew how to be thrifty, but his ability to budget was kind of weak. HOWEVER the flexibility to make changes and chose materials was WELL worth it to me.
Make sure that the bldr has a line item for "Punch" or "misc" or "contingancy". I've had a handyman at my house for two weeks punching stuff out, and that's not in my budget at all.
I did my of "trash removal" and saved around $2500, BUT I have a residents-dump-free dump available to me, and could burn the framing scraps (ALOT of scraps!) This was harder than I expected.
I did the low voltage wire. I saved maybe $1500-2000, but in some respects, I know I made mistakes that the pros would not have.
I did some interior cabinetry, and the mantle, and saved about $10k, but this is was really funny money savings since I wouldn't have really paid anyone this much money to do it, if I hadn't. I would have gotten lower quality stock cabinets and mantle for under $1000.
And I'm doing landscape/seed. The budget was $5k, but I don't know how much of it I would have used.
And I did a bunch of handyman type stuff, and run-around somebodies-go-to-it stuff.
None of these projects endangered the schedule (well that cabinet could have, so I was under the gun for that). And honestly, thats all I could really handle, maybe it was too much. It was rewarding, but I can't wait to wrap it up, I'M EXHAUSTED.
Good Luck,
Mike