Generally I have avoided those to which I could ahve gone. The last 10 years were with a company with 600+ employees. The very first year we were split "north and south", so only around 300 at each, and the Marriott at Slough was big enough not to be cramped and managed the catering well. But still and expensive night for mediocre food and raelly really crap music. The following years were amalgamated into one do, first at a hotel near leamington - way too small for the numbers, trestle tables of 24 people, so converstaion was limited by and to your immdeiat neighbours. After that the Metropole at Birmingham airport. Lunchtime burger £18.00. We went once, and after that I lied to my wife to prevent re-occurrence! Last year was in Dublin - a small friemdly company, but still one where a "night out" has to end with people so pissed they can remember the next day what they did. I didn't go, thankfully having the excuse of returning to the UK for my wifes school do - which we also didn't go to. Do you sense a theme here??
They can obviously be very pleasant experiences for some, but most larger gatherings are let down by indifferent catering (at best), crap (temporary) staff, and over loud, distorted '70's music - YMCA being a typical "must play" selection from the DJ.