Thought that I would offer my 0.02. I own an IT business and routinely setup my clients with hosted Exchange accounts and use one myself.
I started with the built in HTC mail program on my Incredible, but coming from Windows Mobile I really missed a lot of features. You can schedule times when push is enabled, but you can't choose folders to sync, nor can you search the server. I can't even remember if I was even able to change my out of office settings.
I ended up running the trial of TouchDown, which was the full version other than being stuck with either no Signature or the default "sent from touchdown..." sig. This program has all the features of Wondows Mobile + the ability to access contact subfolders. iPhone is unfortunately and sadly the only other platform that has this ability.
I contacted their support with a question and got a real person who quickly responded.
Touchdown does take longer than HTC mail to do the initial sync, but probably equal to WinMo.
I definately suggest this program if you are on Exchange and it's worth the $20.
Hope this helps you guys! Let me know if you have any other questions.
Edit: I meant to add that I have been on the $30 unlimit data plan for quite a while. I don't know why they have the more expensive plan other than for those who don't know any better.
Also the HTC mail not being able to select folders to choose means that every time you go to sent items for example it will at that time sync and you have to wait for it to populate. That got very annoying!