I've been ambivalent about Fix-NICS. Background checks are a flawed premise, particularly as implemented. But they are what we've got and having something in place is a lot more effective at holding off worse gun control measures than not having anything at all.
Doing it this way -- attaching it to must-pass legislation without significant public debate on the merits -- is a classic smoke-filled back room political move. I don't like that much even when it's supposedly our guys doing it, and I don't like giving the gun control crowd anything for free.
I'm not sure whether I should be opposing this or not, but I do think it needs to be known so the gun community can decide whether to be angry about it.
UPDATE: House Freedom Caucus will introduce amendments to the omnibus. One of them would strip the Fix-NICS language, arguing that it was merged with the national concealed carry reciprocity bill and needs to be passed together with that legislation. I'd support that; it has benefits for both sides. Other amendments defund grant programs for sanctuary cities and defund planned parenthood.