As mentioned above, it really depends on your group. There will always be people there who are only doing it for the cert and do the minimal contribution and pretty much, achieve nothing. Even though there's rumours that people can "get the sack", I've never seen or heard it actually happen. That being said, it does teach you a lot about the real world of business- yes there are lazy people who do nothing and you can't get rid of, but there are also some amazing people out there who whilst appear absolutely normal, or even lacking in areas (eg. not too smart), you'll realise that when they put in all their effort and hard work- you'll appreciate them a whole lot more.
In terms of the actual practical experience, YAA is quite good because it does force you to do things in the real world. Sure it isn't exactly realistic for a few factors, but it's as close as you'll get without taking on a whole lot more risk, effort and worry for a year11 student. If you put in a lot of effort, you may not realise it but you'll learn a lot- may not be tangible or even recognised, but wait until you have a job interview and have to explain what you actually did and how it can be applied to other environments. Also wanted to add that even if you don't get a position in the group/team you wish to have a career in (eg. Accounting/Finance), don't worry too much because in the end, people always help each other's division out (unless your team is small and crap).
Anyhow in conclusion, if you have an interest in running a small biz and want to try a 'much more realistic' experience, I highly recommend it. After all, at 2 hours or so a week, with the ocassional trade fair or overtime, you don't really have much to lose besides a small fraction of your time, but what you stand to gain, is priceless.
