I can't say I'm a fan of BNS, I've visited over many years from the old station to the new and it's never been great. Whilst they've certainly improved the above ground atmosphere and the massive glass roof certainly makes things feel a lot more airy.
My biggest issues with the station are, as many have said, the wayfinding, but for me it's usually the way finding to get out of the station in the right direction. Because the station is so big and you can't see out / where the platforms are it's hard to know which way is north in the station and so finding the right exit is often a 5-10 minute walk around the station trying to find the right one and nearly always realising I've headed towards the wrong one and the one I need is all the way back where I was. The signage absolutely does not help, if you're not familiar with Birmingham the "Bullring" exit is likely the only name you'd recognise.
Also, if you're trying to find a lift to get a buggy down to the right platform... well, the signs are AWFUL for this!
It's obviously working well for some regular users but certainly when I've used the station I've heard many, many, many confused passengers. Someone quoted Stratford earlier and I'd agree that is a hodgepodge of a station, but the one thing that makes Stratford easier is that you don't actually have to care about the platform number, you're essentially just following the sign that points towards the DLR or Central Line or Lizzie and so you're essentially just following a coloured strip which is really easy to do.