I have been using the 266xs for about 18 months now. I run it as part of my standard bass rig, with the bass going direct into the first compressor gate and the second as part of the effects loop.
The 266xs works exactly as you would expect a compressor to. Each compressor has it's own noise gate, allowing you to run 2 feed through it completely independently. Each gate can be turned off if required by simply turning the threshold down to the off position, and also features the standard ratio control to control the effect. That is the extent of the Gates functionality however, so if you are looking at this as a noise gate first, I would look elsewhere.
Each compressor has the standard range of dials, Threshold, Ratio, Attack, Release and Output Gain. Each control has lots of range, and the dials are nice and firm so you can be really precise with your settings. The threshold and gain reduction led's are nice and bright and quick to react so you can see exactly where you are being getting the benefits of the unit, plus they jump around and flash while you play which looks cool on stage.
It also features DBX's Overeasy technology, which in all my experiments with it, does nothing. It is a similar story with the Autodynamic function, this does work but I personally have never liked its effect and prefer to leave it off. The basic compressor though with none of the bells and whistles turned on is great.
2. Sound quality
The output sound is very clean, you can hear the compression difference but it doesn't get that weird electronic 'squelch' (That's the word i'm using and i'm sticking with it) unless the compression is very heavy and/or the signal in is very loud.
The gates have minimal noticeable delay if you suddenly smack the strings too hard, not the best I've ever heard, but you cant complain.
3. Build quality
This thing is so well put together. the controls feel nice and firm, the buttons click nice and reassuringly, and all the led's still work after 18 months of pretty constant use.
The rack unit this is mounted isn't top end and has had a few knocks but the DBX has weathered if all.
For the price I think this is a great unit. Yes there are cheaper pedal compressors out there and yes there are better rack mounted compressor but this sits between the two perfectly. DBX have rather over-hyped the over-easy in my opinion but the core features do exactly what you want.