TRUCE/STAC
I've been to a few sessions at Swanwick, and quite simply, they should be mandatory for all professional aircrew ! Everyone should badger their management to build in some time to the rosters to allow for a day for pilots to attend. Pilots would get more out of that one day, than the mind numbing hours doing the CBT, that passes for recurrent training these days - IMHO !
STAC is probably more beneficial to pilots, as it's a day of debate and discussion, directly with ATCO's. However, TRUCE uses pseudo-pilots, and is not only interesting (and also with the opportunity to listen in/contribute on the de-brief), but is also useful in being able to provide info about decision making processes, technical knowledge in specific aircraft, prioritising, checklist handling and flightdeck managent, and all the good stuff we do in our simulators etc etc etc.
I've been a professional aviatior for over 16 years, and wish that I had had the opportunity to have this kind of experience at the beginning of my career, rather than at the end, but even so, we never stop learning.
JUST DO IT !
You won't regret it, the grub's good too (airline caterers take note !), and not only will you be made to feel very welcome, but you will be also treated like a professional, and your opinion valued - which is quite a rare thing for pilots these days ! And you will be impressed !