I've been using Exchange 2007 for probably 18 months or so however we completed the Outlook Voice Access and Voicemail integration internally only recently in the UK.
I am really impressed and amazed with how powerful Outlook Voice Access really is.
If you don't know what Outlook Voice Access is - it gives you the ability to dial in to your Exchange server and have it read your email, voicemail and calendar. You can also look up contacts on the Global Address List but my favourite piece is processing email via voice!
I have an almost 80-90 mile commute to our Reading office and although I'm not in the office everyday it's a fairly long commute each way. Now obviously I cannot do my email on my device whilst driving as it's illegal and infinitely dangerous but..
What this means is that by the time I reach the office my mailbox is completely prioritised and I have got through the 30-100 emails I get overnight from around the World.
There is a whole host of functionality beyond this with the ability to cancel meetings and even inform people you are going to be late for a meeting. I was presenting at an Executive Briefing at our offices in Reading a couple of weeks ago and got caught in traffic so I just called Exchange and said 'I am going to be late' - it asked me by how much (I checked my Windows Mobile device Sat Nav for my ETA) and Exchange then sent the meeting organiser an email telling them I was running 15 minutes late.
Outlook Voice Access is just another reason why Exchange 2007 is such a massive leap forward and if you are trying to justify this internally I suggest showing your internal sponsors Outlook Voice Acccess. Everyone I demo it to... wants it!