A few weeks ago I switched my trusted Nikon D800 for a Pentax 645D - my first ever medium format camera. Whether or not there is an image quality difference has been debated and probably depends on the usage. I believe D800 or another DSLR can't be beat in terms of a combination of ease of use, high ISO performance and image quality. But I do feel Pentax 645D when used in a slow deliberate manner, on a tripod with high quality lenses does offer better image quality. This however requires a lot of work to get and see that perceptible difference in image quality though.
Where as with D800 I used to be able to put a high quality lens like Zeiss 21mm Distagon or Leica-R Summicron 50mm or Summilux-R 35mm and pretty much know that I will get a superior result, with Pentax 645D if I am not careful results can be under-whelming and sometimes even poor. Only when using the best practices of low ISO, very sturdy tripod, mirror lockup, does the image quality in terms of sheer image detail, color, look etc. surpass the D800 results. The difference is certainly there its just not easy to achieve that difference.
I hope with time my technique will improve and the better technique will become second nature and the image quality will get consistently better.
So for now I'd say its worth it but its making me work much harder than before.