I've been a user of DxO since Optics Pro version 3, I think. I tend to skip versions so it's unlikely that I'll be upgrading my current Photolab 1 to version 2. I use it as an alternative to LR. I can usually get what I want in LR, but sometimes I switch to DxO to give myself a different starting point.
There are somethings it does very well:
Lens corrections for vignetting and distortion are excellent
Capture sharpening is usually excellent. Has been for my lenses and camera bodies. Have seen some sample RAWs where the DxO sharpening was atrocious.
Highlight recovery, if not blown too far, is excellent. LR is better at extreme recovery
Noise reduction really great too. I tend not to use PRIME, as I don't like the look it generates, and I don't shoot much extreme ISO stuff anyway.
I like some of the film and grain simulations too.
I also like how it can retain variation and detail in highly saturated colours. It obviously has to desaturate to achieve this, but the control of the process is good.
Photolab 2's support for dcp profiles is something I'm keen to test, but haven't gotten around to just yet. Might make it easier to get one's familiar 'look' in DxO.
Can't comment about performance on Mac. I'm on Windows and DxO has always behaved itself. Output processing has always been slow, but steadily improved over the years.
DxO can certainly output a ProPhotoRGB file, but this will be limited initially by its internal working space. I've heard that the internal working space is AdobeRGB / something similar to AdobeRGB, but I've never seen it clearly stated anywhere. Anyone got a reference for this?