I'm very pleased with the v4 flannel mask. The previous masks I've primarily worn were the v1 mask and some surgical masks.
The v4 mask was very formfitting; more than any other masks I've used, I noticed that it often sucked in/out as I breathed, showing that it had a good seal around my face. The adjustable nose piece was far superior to the plastic nose piece in the v1 mask, and held its shape well. The flannel fabric was softer and more comfortable than the material from other masks I've used. Overall, its the best mask I've tried.
However, the nice sort flannel fabric was also the main problem I experienced with the mask. It was very warm and when I was breathing heavily, it felt like a greenhouse that trapped my hot breath and amplified it. It also seemed to take longer to dry; and often leaving it hanging overnight to dry wasn't sufficient. Finally, the adjustability of the neck strap was awesome, and the ability to take it off and let it dangle from my face was very useful.
Final judgment: despite the uncomfortable warmth of the mask, it felt much more effective and comfortable than other masks. I'd gladly buy more if needed. 4/5 stars.