I've been a few times and the obvious main negative is that you're a long way away from the pitch and the other one is that the atmosphere is nothing like the Boleyn as they've now attracted the middle classes and families like Arsenal et al. I've been in the upper tier of the away end behind the goal and you need binoculars to see the action in the opposite goalmouth, I've also been in the home end alongside the pitch and, whilst you're nearer the pitch, you just can't get a very good perspective. The upper v lower decision is like any other ground but just add around 30-50m onto what you're used to. Think of the old Wembley. I think upper tier is still the best personally, the nearer the front rows the better.
The positives are that the facilities are great: lots of space and large concourses with lots of choice for food and drink albeit expensive. In terms of drinking and eating before and after the game the London Stadium is one of my favourite grounds in the country. On the east side in Stratford you have the traditional pubs favoured by the old skool fans (probably best avoided as an away fan) and on the north-west side you have the River Lee and Hackney Wick which are full of places to eat and drink. It is all bit hipstery - think craft beer, bars on boats, bars with board games, bars with space invader games, vegan options, etc, etc - and a lot more friendly.