Mine just came earlier this afternoon. Had a little bit of time to do a comparison against the Modius.
Initially, I upgraded from the Magni/Modi stack to the Asgard/Modius stack. The step up was a big difference to me. Things were more defined, bigger soundstage, better imaging, etc (Although, I do think the Asgard 3 had a big part in the SQ leap). As a result, I wanted to see what the upgrade to the Bifrost offered, so I ordered one a few weeks ago.
I'll preface by saying I don't have a lot of experience with various DACs. Basically, it's been a Schiit journey (Fulla->Modi->Modius->Bifrost).
To me, the Bifrost does sound better than the Modius, as expected. The sound is a bit smoother, better bass extension, and an overall slightly fuller sound. When I switched back to the Modius, the soundstage felt like it was a little more front loaded and just a bit narrower. The Bifrost is actually having me reconsider selling my Grado GH4s since the highs on them were just a little hot for me on the Modius. I'm really enjoying the BF2 so far.
I will also say that the Modius seems even more so like a very capable DAC at $200. Does the BF2 sound more than three times better than the Modius as the price would imply? Personally, I don't think so. This is in no way a knock on the performance of the BF2 since it's obviously an amazing DAC, but rather a nod to another Schiit product with extreme value. The difference between the two is definitely noticeable and if you are looking for a DAC that will last a very long time, then the BF2 is it.
I don't have any plans to upgrade and I know that the BF2 will be very capable in bringing out the best in any headphones that I may get in the future.