Using beta software with existing data always generates a caution to back up data and not use the beta on production systems. Most people ignore this advice and most of the time there is no harm, no foul.
Do not use beta or Preview software if you are unwilling to accept glitches or require specific features. Bugs can occur and previously working features may stop working as a result.
Occasionally there will be problems – a major problem for Outlook 2010 users is the inability to sync with Smartphones. Outlook 2010 64-bit won't sync with any phone, while the 32-bit version works with ActiveSync but not BlackBerry Desktop Manager or iTunes. Data is not compromised but if syncing with a device is important, don't use the beta software. If you have add-ins you can't live without, don't use the beta software before confirming the add-ins you need work with Outlook 2010.
Brian Tillman, Outlook MVP noticed a problem sharing a PST between two versions of Outlook and syncing with a PDA:
"A caution in using a PST with both Outlook 2010 and an earlier version
I have a dual boot PC at home. One partition uses Windows XP and Outlook 2007, with a PST kept on a second drive. The other partition uses Windows 7 and Outlook 2010. I reference the same PST in that version of Outlook as well. This works fine within the two Outlooks until one tries to sync with a Windows Mobile 2003 PDA running "Microsoft Pocket PC Version 4.20.0". I sync the Windows XP/Outlook 2007 installation using ActiveSync and I also sync that same PDA with a Windows XP/Outlook 2003 system at work. I've been doing this literally for years, with, first, Outlook 2003 on the home system, then with Outlook 2007 beta, then the released version of Outlook 2007 with no problems, prior to accessing the PST from Outlook 2010 as well.
Since I started using the PST with both the Outlook 2007 installation and Outlook 2010, new calendar items added on the work PC in Outlook 2003 and synched with the PDA will not properly sync with Outlook 2007, stating that the form contains improper values. Non-calendar items sync fine. I conjecture that Outlook 2010 has altered the calendar records form in such a way that doesn't bother Outlook 2007, but does bother Pocket Outlook. I don't expect anyone to solve this problem - I'll probably solve it by giving each Outlook its own PST again - but I wanted people to know about it."