Malware packers

Quite a few pieces of malware, be they viruses or trojans, now use packers to obfuscate their presence. They also use them in an effort to bypass anti-virus software. The process of reverse engineering malware that use packers such as PEX, Yoda, and ASP to name but a few, becomes all that harder to do when packers such as those mentioned are used. It is not impossible to do so, but for the inexperienced, or beginners, packers are one serious pain. Do any of you have any stories, or tips to share that involve packers?