I bought the Professional 600 (lift bowl in glossy gray) about 15-20 years ago. I bought it at a Linens & Things during an incredible sale. The reason was a cookie I make every year that is more like a candy with 3 flavored chips and 3 kinds of nuts. I literally could no longer stir the batter at the end. This mixer mixes it!! I can't do it quickly but the power in this machine surpasses every machine I've ever owned. I'm still doing this cookie every year! IMHO there is no comparison between Kitchenaid's tilt back model and the Kitchenaid's Professional 600. It is the absolute workhorse of mixers out there. Like I said, I am still making this cookie and it is a BEAR to make, but not with this model stand mixer!! If you are lucky enough to buy one today, you won't be sorry and I think it is very much worth the price!
BTW, I thought by now I'd have to buy another Kitchenaid mixer!! But no, my mixer is still working with no problems and my cookies are loved and looked forward too... Yay!
*~"Never eat more than you can lift......" Miss Piggy~*