I've been experimenting with the water soluble oils a bit, since some of my students for the Beginners Online Oil Painting class will be using them. The apple and the bottom piece were done exclusively with the Artisan Oils (water soluble) by Winsor Newton, and the little diptych at the top of the post was done with a combination of the water soluble and the traditional oils.
The brochure for the starter set I got says that yes, indeed, you can combine these with traditional oil paints, substituting the special Thinner and Mediums for your normal OMS (Odorless Mineral Spirits) and whatever medium you normally use with your oils. I found the combination to be a bit difficult, my paints got tackier than normal. Or too slippery.
Here's what I've found so far:
I had to work harder to maintain my values with the water solubles. The paints seem to want to mix rather than lay on top of subsequent layers, as I'm used to.
The W.S. paints in the lighter colors seemed chalkier for some reason than my normal oils. Some of them were thick, some thinner out of the tube, but that's pretty standard for normal oils too. So that doesn't count really.
I will try some of the Duo Aquacolors by Holbein, which are supposed to be of better quality than the Artisans. I'll do another report when that happens.
I think that for people who can't tolerate the smells associated with traditional oils, these paints are a boon, and if you worked exclusively with them, you would get to know the capabilities better than by doing three little pieces. For that, I think they are wonderful. But I still prefer my oils, maybe since that's what I've gotten used to.
The final verdict is in the individual use. I think that if you try these and like them, then that's the proof in the pudding.