At first I was happy about Lambda in PHP 5.3 but now after several months of developing with 5.3 in in my day to day work, I found that I rarely use Lambda in PHP. Unlike JavaScript where I use closures ALL THE TIME.
The really most useful feature for me in 5.3 is late static binding. Almost every time I have to develop something in 5.2, I really miss it.
And just to make it complete: The worst idea for 5.3 is GOTO. 'Nuff said.