A bunch of issues have been reported in regards to the new iPhone 2.2 Firmware upgrade. There is a list on iphoneatlas.com’s site. Here are two of the biggest issues at the moment.
Third-party Applications broken Some third-party applications will no longer function properly under iPhone OS 2.2 due to API changes and other modifications. These include a number of applications that make use of CoreAudio. These applications will generally need to be updated by their developers. However, you can try uninstalling then reinstalling applications for problems that do not involve API or other strict compatibility problems.
“Could not Connect to Internet” Some users are encountering an inability to connect to the Internet over cellular networks after the iPhone OS 2.2 update. In some cases, this issue can be resolved by switching from 3G to EDGE (or, for carriers other than AT&T, the appropriate GPRS network).
The full list of issues can be found in the article, Users Face Serious iPhone OS 2.2 Problems
At the moment I’ve held off on the update as I want to be able to Jailbreak my iPhone. At the moment its on the 2.1 firmware not jailbroken, why? Well because I had installed so many applications it just got slow. So depending I may not even Jailbreak my iPhone again, I rather like how speedy it is.