I hated it at first, and when it really took off as the trendy thing at least here in my country I particularly hated it because they were outrageously sweet. It was like having a burger between 2 slices of cake, it sucked. I also felt there were textural things that just weren't right and I complained about the hipster takeover of good burger bread.

I've mellowed on it now, I think in part because they've actually changed. I think the commercially made ones used in burger places now seem to actually taste of bread and are only just a little sweet and the whole combo especially with lots of mustard works really nicely. They look beautiful and when they aren't super sweet they add a little something without being too cloying or distracting. I appreciate nice flavourful bread in a burger but ultimately it's a vehicle and brioche strikes a good balance between the awful grocery store bag of fluff burger buns and super hard chewy hipster sourdough or some weird, not round form factor bread that should really be a pita or a pizza. So long as they're toasted, they're all good and it grows on you. Which is fortunate as everybody seems to have decided that that's burger bread now so I'm glad I picked up the taste for it.

I also had the same thought on the greasiness but then I kind of discovered how much nicer the super greasy, drippy, messy kind of burgers are and once they're made like that with tons of juice and fat, they're so greasy and messy that no bread is going to save you from having completely greasy hands anyway so some negligible amount extra from the bun isn't all that worth worrying about. If it's one of those burgers with the tighter texture that's not quite so indulgent, maybe a bit drier, not as big a pattie then the bread is a lot more important and the Brioche is a less good option, especially as it's also greasy but otherwise, I've changed my tune on the brioche.