I never used any other then DJM600/DJM700 mixers. Honestly I can't see much difference between them, perhaps a bit on sound front where DJM700 may sounds better.

I own an used DJM 600, 10 years old, one of the first of its kind. It still work flawlessly after 10 years. I own it for less then a year but already it survived some accidents. Recently on party somebody spilled sticky strawberry juice over it and it just proved its quailities since it is hard for liquid to get in thanks to protectors on sliders and inner plastic protectors above the electronics. It is very well build, quite resistant to surface scratches or any usage marks like broken knobs and so on. Furthermore it is easy to dissasemble the device and replace all movable parts (knobs, faders). I am very pleased with this unit and I am not looking to replace it (perhaps this is not a good tactics for Pionner to build such a bulletproof units because users won't upgrade them:) Regardless its age it is still among the top mixers and in my opinion it is still better option then many new mixers from other manufacturers.
PS: I think when people talk about the bad sound quality they do not realize that they should not go into red areas while amplifying the signal neither on channels or master, the sound should be amplified on amplifier, if you go just two steps into red zone on volume meters sound will be great in my opinion. Try to find problem in your amp/speakers and distortion rather then the mixer.

I paid £560 for a new pioneer djm600s, after owning many cheap mixers in the past I decided that instead of buying every 2 years I would pay more and get something to last a lot longer.
1st day I set the mixer up I dropped a metal mic onto the top panel which bent the crossfader ! Fortunately I bent it back with pliers and it has continued to work 100%.
The rotary knobs & switches do feel very robust, although the upfaders and crossfader feel slightly flimsy in comparison. The mixer is great to use, no channel bleed noticable, low background noise, and loud headphone output.
The effects are very good, and all of them can be used effectively (something which was a problem on my previous mixer - a behringer djx700), and the sampler is fantastic.
The actual output audio quality isnt as good as I thought a mixer of this price should be, its good but somehow not as amazing as I thought it would be considering the price. I would have liked kill buttons for the low/mid/high frequencies like on my old djx700. Overall it is a great mixer though and well worth the money.