Synchronising Windows Mobile with Outlook can often be a game of chance - sometimes it will work smoothly, other times you will lose contacts, tasks or appointments on one side or another.
This is particularly a problem if you are synchronising with two PCs; my personal experience has been that every time I switched PC, I'd have to disable and re-enable at least contacts and calendars every time. From my research into this issue, it seems to be fairly common on Windows Mobile 6.1 particularly.
PocketMirror offers an alternative method for synchronisation. It still works through ActiveSync - which means that there is no need to run an additional application after connecting your phone or PDA - but the PocketMirror application adds new entries for PocketMirror Tasks, PocketMirro Calendar and PocketMirror Contacts. These are used, if required, in place of the standard entries in the ActiveSync options, allowing you to switch between tools with ease.
PocketMirror seems to do a far better job of identifying changes correctly; since switching to PocketMirror, I haven't had a single failure on synchronisation.
Additionally, PocketMirror offers the option to add folder names in Outlook as Categories in Pocket Outlook. This can make managing a large collection of tasks or contacts far simpler on your PDA, without having to change your way of working in Outlook.
The speed of synchronisation is about the same as with standard ActiveSync - it seems slightly slower, but is infinitely more reliable, so overall it's probably saving quite a lot of time.