Just like both Mark and Chris said, the products differ too much to be compared with eachother, and now, I will make it more "unfair" because I will compare some differences between TMG SP1 + Software update 1 vs BETA version of pfSense 2.
pfSense is an incredible firewall with many nice and advanced features, and extremely low hardware requirements compared to TMG. It´s free and is updated rapidly. It is easy to use and responds very quickly to your settings.
pfSence seems to handle enormous blocking rules FASTER than TMG and you can notice that in traffic flow in both ends of the firewall (after the packets have been processed) and you will also notice the GUI-response to be pretty fast on pfSense compared to the very slow TMG. Have not tried load balancing for this test.
When it comes to stability and reliability, pfSense does not seem to have any chance at all. We made a stress test with a setup of 10-machines with 8-torrents each plus 2-ftp servers and a SharePoint farm with 2-nodes behind a firewall connected to the Internet via 1 Gbit fibre. Additionally, we had 6 other machines connected to Internet via separate 100 Mbit lines (different locations with 4-different ISPs) to disturb the traffic which was flowing through the firewall. Undisturbed, the traffic flow through the TMG firewall was pending somewhere between 60-120 MB/s (upstream and downstream) and with full attack from all 6 "disturbers", the throughput speed went down to 30-70 MB/s. The webpages on the SharePoint server were responding pretty fair during the attacks. The same test with pfSense as firewall was horrible :-( The throughput went down to 2-30 MB/s and most of the clickings (from an external network) on the SharePoint webpage timed out. We even changed the hardware to another manufacturer to be sure that it was no compatibility issues with pfSense and the server(s), but it was hard to tell if it got better or worse.
I do not agree that TMG has minimal functionality for use as advanced NAT, routing, multiple WANs etc; pfSense offers more, but absolutely not MUCH more.
For use with Windows clients on the inside, TMG offers a lot more functionality, especially if combined with the rest of the ForeFront family.
Reports and monitoring in TMG is much better than pfSense.
If you really want to compare TMG with another firewall, try Cisco. I like the combination of ASA and TMG.
If you intend to stay partner with MS for a long time, I suggest that you use your partner benefits to its limits! ;-)