IDG Answers is a community of experts who are passionate about technology. Ask a question or answer one below.
You may want to hold off on releasing SP3 if you're in the middle of some kind of development project, but it's a pretty essential upgrade. I've installed it on a couple of servers with not much of a hitch, except for the reboot required.
From Microsoft's own notes, "Exchange Server 2007 SP3 is a mission-critical communications tool that... provides further flexibility... along with enhancements for the advanced protection options against e-mail security threats, such as spam and viruses and the tools which help manage internal compliance and high availability needs ..."