The 5 ilevel boost to all gear from BRF goes live tomorrow, and it seems most everyone is OK with this. The most obvious effect will be the effective nerf of all content.
I think that people making good progress through BRF on any level of difficulty will be boosted significantly, while others who have not progressed much are likely to still be using more HM gear, and so will see less immediate benefit. However, I think that in 2 weeks, the new ilevels will be the new normal.
Some are saying that this ilevel boost makes BRF a separate tier from HM, but I disagree. Consider the following graph of MoP ilevels:
As you can see, MSV is in the same tier as HoF and ToeS, but at a slightly lower ilevel. The overlap is significant. Between each tier, however, only heroic and LFR overlap. The tiers are distinct, and all gear from old content is clearly obsolete for a progression raid team.
Now, the argument that HM and BRF are separate tiers seems to be based on the full replacement of HM gear with BRF gear, at whatever difficulty level a given raid team is progressing at. I argue, however, that this is expected, as the same occurred in T14.
Also, HM remains relevant for many raiders if they were to increase the difficulty of HM. Suppose a raid ream has cleared 6/10 normal BRF after clearing 6/7 normal HM, and a few heroic HM. They would likely be at 665 ilevel, so heroic HM still drops upgrades that are now only equal to normal BRF, not 5 ilevels above. This is a mild nerf to HM gear, but it is not made clearly obsolete.
Another point of contention is that Imperator is now obsolete, and I'm inclined to agree, but only to a degree. Considering the same raid team example, downing normal Imperator to complete that progression does not yield upgrades, expect perhaps a few BiS items where a lower ilevel outperforms a higher one. However, this is a niche example.
All in all, I think that in 2 weeks none of these complaints will be an issue. The ilevel boost is justified on it's own merits.