Don't believe all the reviews you read. It may be a case of: Celsius vastly improving their graduate assessment day in a short space of time - or - people feeling a bit miffed about how they were dealt with when given no offer.
The day is informative, so go, you learn a lot. If you love sales I guarantee they'll put you in a fantastic role and you'll reap the benefits of the industry and sales sector.
However, the day is at times quite odd, you are told exactly what to say before you go when there is a question & answer session. The activities are thought provoking, just follow your talent manager's advice. You'll then receive lots of advice and tips for interviews and assessment centres. You really learn a lot and they're focused on preparing you extremely well and placing you into a role ASAP, so be prepared to throw yourself into it.