There is some interesting dialog in customer reviews on Amazon, maybe this pinpoints the original problem Shel was asking about (check out the negative reviews, there's a detailed review by an engineer). Raises issues about a plastic gearbox, sounds like this might have been resolved, I'm not sure.
Amazon.com: Customer Reviews: KitchenAid? Professional 600 6-Quart Mixer Blue Steel
Lana, my aunt just bought a Pro at Costco a few weeks ago, the coupon was $100 off, not $75 like I thought. The Pro at Costco comes with full metal beaters, solid metal dough hook, says it's all metal construction.
Costco - KitchenAid 5-quart 475-watt Stand Mixer=
Lana, if you're going to bake bread, IMO you will need a strong motor and full metal beaters that the Pro lines offer.
Now Costco's pro is 475 watt, there are some Pros that are above that too. I understand the Pro600 as KYH mentioned is 575 watts.
My old Pros are only 350 watts, but solid. I did manage to ruin one of them years back after much abuse (hard dough at 8-10 with a dough
hook:blush:), but in general, they've been standing up great, haven't had any other problems, haven't had a need to replace.
I would say if you're expecting it to take serious abuse, check with the manufacturer as to which specific model doesn't have that plastic gear case those negative reviews are referring to.