They say it better than I can...
Just want to point you to a few blog articles I have read recently just in case your search doesn't reveal them or if you are lacking a bit of reading material...
Introducing a New Era for ISA Server (interesting info about FTMG [Forefront Threat Management Gateway])
A Look at Threat Management Gateway
Troubleshooting OWA 2007 Publishing Rules on ISA Server 2006
I know everyone reads the exchange team blog but this is worth pointing out - very useful bit of information for enterprise customers spec'ing Hub Transport Server (New maximum database cache size guidance for Exchange 2007 Hub Transport Server role)
..and where's DPM?