I've started evaluating TD, Moxier, K9 and Roadsync and it seems that there are several criteria that you need to consider whether they're important to you.
1. First off, you can turn off part of the regular exchange sync - and only have part of the the complementary program sync the other parts. why is this?
2. Contacts - this could be a bit problematic if you have alot of contacts - with 10k contacts, duplication is a problem on internal space. I've found that Roadsync syncs contacts similarly to the stock app and doesn't change anything. Touchdown, on the other hand, disregards the category function in outlook and re-downloads all contacts into a touchdown category - so when you're viewing a contact in a native app like people, you're looking at your outlook contact whereas, if you're looking at the contact in touchdown, it is the copy.
Mail - subfolder sync - i organize my mail into subfolders, so this is an issue - i also don't need to have all of the folders synced on my phone - only the pertinent ones. The stock mail app is awful with navigating to the subfolders and will only sync the subfolders when you access it. Not cool.
Roadsync and Touchdown have some nifty features in this regard and both are more a matter of personal taste. Both allow you to specify which folders to sync (as do k9 and moxier but i haven't spent much time with either yet). I found specifying the folders much easier in touchdown - similar to the iphone checkbox compared to roadsync that asks you to toch a little gear-like widget that is too sensitive, and sometimes gets the folder wrong.
As for viewing the mail you're syncing, i found roadsync does a better job in presenting the combined mail in a folder called "all" rather than inbox and it's very easy to switch; touchdown is a bit more complex, but has better color scheme.
I haven't found one i really like - which is to get a widget of the stock agenda view. Whereas Touchdown had its own internal contact storage, roadsync has its own calendar storage - which makes it confusing as to which app to sync.
A couple of things I have yet to see:
The ability to store everything on the SD card. Since your phone is syncing to exchange, there is less concern about losing your content - thus, you should have the ability to offload everything from internal memory and free up space.
Better scheduliing using the calendar - especially when inviting attendees. I'm sure this will be forthcoming.
As for Moxier, i found each of their apps are separate, whereas you can go to a unified screen for roadsync or touchdown, or select a specific action, or widget to launch the specific action.
The summary so far? I personally haven't found one that i really like enough to keep - all have their warts and all some niceties. But, in honesty, they are all better than the stock Iphone exchange app which totally sucked.