Here are some experience regarding management packs from a side by side migration between MOM 2005 and Ops Mgr 2007.
Exchange 2003 MP
There is problem with Exchange APIs handling the mail flow check. It looks like both MOM 2005 and Ops Mgr 2007 are sending mails, but only MOM 2005 recive them. The other component of the Exchange 2003 MP is working fine.
AD 2003 MP
You cannot monitor a domain controller running 64-bit Windows Server 2003 with Operations Manager 2007 and Microsoft Operations Manager 2005 at the same time, because the MOM 2005 agent running on 64-bit domain controllers is the 32-bit version of the agent, but the Ops Mgr 2007 agent is a 64-bit version. The MOM 2005 agent contains the 32-bit version of the Active Directory Helper Object (OOMADS) and the Ops Mgr 2007 64-bit agent cannot use the 32-bit OOMADS. In other words, if you have 64-bit domain controllers you will have to decide when you switch them over from MOM 2005 to Ops Mgr 2007.
There was no bigger problems found within the Base OS MP, DNS MP, IIS MP, Cluster MP, SQL MP or DHCP MP.