It's been a while since I shopped for one, but I think I recall the Pro-500 getting a lot of negative reviews as far as reliability. But the Pro-600, I think, was lauded for being a real workhorse and worthy of the KitchenAid reputation of old.
I don't think, personally, you could do better for the money than this if you were looking for a large stand mixer. Which does lead me to state the obvious: it IS large, and perhaps too large if you're not churning out batches of gruel for the orphanage 3 times a day. I have a large and a small KA, and with just me and the wife in the house I find myself plugging in the 4 quart more often than the behemoth. It just works better on things like kneading a single batch of bread dough, or making enough whipped cream for just one 9" cake.
Still, when you DO need massive firepower, it's good to have the Monster Truck of KA's at your disposal. For anyone who spent time using the classic Hobart restaurant line when the two were owned by the same company, you know what I mean. This Pro-600 will probably be as close to the small Hobarts as you can get now, at least in the KA line.
http://www.ebay.com/itm/Kitchenaid-KP26M1X-Pro-600-Stand-Mixer-6-qt-Super-Big-Large-Capacity-Many-Colors/360593721010?afsrc=1&rmvSB=true
Mr. Enabler, out.
