Using PNF in treatment of stroke patients should improve e.g. the gait. By using PNF you can stimulate normal movement patterns; by 'stearing' the movements and therefor having the ability to inhibite high tone muscles and stimulate low tone muscles. If you look at 'extension and flexion patterns of e.g. the leg in stroke (most common patterns) you will find in PNF that certain PNF patterns move away from the most common stroke patterns. In case Wikipedia reference-linkBobaths reasoning is sound, the likelyhood that PNF does work is large. With PNF you can go through the same steps of physical development as with bobath (e.g. crawling, from sitting to standing).
I have found PNF usefull in treatment of neurological conditions, one thing comes to my mind; think about ataxia the use of (minimal, only guiding) resistance could well reduce tremor.
Unfortunately I am unaware of any research but I must admit I have't really looked into it.