Rand Morimoto wrote a great article for Network World about Exchange calendaring issues. Rand is the president of Convergent Computing, the company I work for. His article covers lost appointments, duplicate appointments, odd delegate issues, etc. This article is based on the real-world experience we as a company have gained working with many organizations of all sizes.
I wanted to add a few comments of my own, to follow up on to Rand's excellent article.
Another factor that can cause calendar issues for delegates is Cached Exchange Mode, which can cause an artificial delay in updating calendar information. For example, the boss (using Entourage, which doesn't have Cached Mode) may accept or alter the same appointment that the assistant (using Outlook in Cached Mode) accepts or alters. The default Cached Mode settings may cause a delay of up to a minute before the assistant's calendar item is updated on Exchange. Who wins is anybody's guess.
Regarding BES, RIM is making quite a few changes recently due to inconsistency problems (especially in calendaring). BES 5.0 SP2 is supposed to make huge performance improvements so that BES IOPS are now equal to Outlook (see http://flaphead.com/archive/2010/07/24/bes-5-0-sp2-blackberry-user-exchange-2010-iops-now-equal-to-an-outlook-client.aspx). The rumor is that BES 6.0 will completely re-architect BES to use Exchange Web Services (EWS) rather than MAPI.
That said, BES changes versions as often as we change socks. I find that most orgs are WAY behind on BES versions from the current version, so they can't take advantage of these improvements. And as with any other technology with massive changes, it will probably introduce a new set of unknown problems.
Being that calendaring issues are 99% due to client issues (Outlook, Entourage, BES, iPhone, etc.), I expect that most of these issues will go away as the clients move toward using EWS. The problems documented in Rand's article occur because of the way the clients handle calendaring/email objects. If the client software lets Exchange handle these objects using standard EWS APIs, the problems will diminish/disappear.