I would not hesitate to use Provia 400X in medium format, especially 6x6 or 6x7. If you shoot handheld with a Pentax 6x7 like I do, the extra two stops of shutter speed can make a very big difference. The grain is incredible for a 400 film, in my opinion. If only we had the stuff 25 years ago, I wouldn't have used Velvia at all. I also agree that Velvia 50 especially and most slide films in general do not like under-exposure. Velvia is too warm and contrasty for portraiture in normal lighting, but it is excellent for nature and landscapes. One thing though is to evaluate the shadows carefully. If they are important, then you have to consider using a different film. If you can sacrifice or ignore them, then no problem. Of course nothing prevents you from changing a rendition after scanning, but your basic exposure has to be good, and one can do very little about morbid shadows even with the best of scanning technique and equipment. Astia is a good film, and I (like many) rue the fact that it has been discontinued. If you can still get hold of some, it would be worth it. I used Ektachrome a long time ago, and it was too blue for my liking. The recent ones were much better, but alas, they are gone.