Unfortunately this isn't one of those success stores. But then again if I wrote about those I'd be hitting a few thousand posts a year, and plus they're really boring to write about.
We began the project by powering up some virtual machines and test importing the configuration from ISA 2006 to Forefront TMG 2010, and all appeared fine. The ruleset was there, the VPN configurations were there, and so on. Test data seemed to pass through nicely.
The migration went through and we put the box live, decommissioning the old ISA 2006 hardware. Everything seemed fine until larger quantities of traffic started passing through the box. The logging was showing a lot of packets getting dropped on the floor, but with no source, destination or protocol, active FTP and SIP traffic was also being problematic, and the box would randomly decide to stop passing everything, like the service had stopped. The irritating thing was that it simply wasn't consistent.
After poking into the configuration we started noticing that a lot of problems were evident in the configuration; The domain controllers computer set had entries that were flat out wrong and not present in the ISA configurationThe Web Proxy Auto-Discovery Protocol (WPAD) file was wrongDNS was starting to go down VPN tunnels, but there were no DNS addresses configured on the interfacesAnd a whole host of other niggly issues
After fixing these the box was still randomly dropping things, but as the data flow increased (and not to extreme levels - we're talking a 10Mbit/s leased line here) so did the drop outs. At this point it was starting to become more than an irritation and more of a service affecting problem. I elected to rebuild it with non-R2 Windows Server 2008, and to manually create the configuration from documentation. Although I would've loved to have got to the bottom of the problem rolling back would've been as much of a pain at this point, and the customer was rightly beginning to get fidgetity.
So why non-R2 Windows Server 2008? A couple of reasons; All our other deployments of TMG 2010 are on non-R2 and are stable, we noticed our original test box for this project was non-R2, and there are also rumblings of other people having issues with R2 on a couple of technet threads. Although I'm not 100% convinced that R2 is to blame here frankly we didn't need R2, and I only wanted to do this the once as the whole job needed to be done out of working hours.
Since the OS rebuild and manual build of the configuration, touch wood, it seems to be a lot more stable. No more weird packets getting logged, no more weird FTP or SIP problems, no more random drop outs.
My thoughts on TMG 2010 aren't favourable at this point, but it's not just because of the problems. Ostensibly it feels like ISA 2006 with a few interesting bits bolted on, but unless you require ISA or TMG in your environment, I wouldn't recommend it. There's still no real IPv6 support, without SP1 it feels very wobbly, and for a few features that you might not need its an expensive upgrade.
Realistically you can pull off the same feature set with a different combination of products; a “real” firewall, and an internal proxy server, for example. This isn't to say that you shouldn't put TMG 2010 in anywhere. It does have some very useful features, but just look at your options carefully. Perhaps you don't need to upgrade. Perhaps you may find a better fit solution.