There are things missing like StompingTom said, but there are python-scripts available to restore missing features from 2.49. Although I don't use any of them except for Beveling.
And the new tool operators for modelling are great, compare Spin 2.5 to the spin available in 2.4x!
The first time I started using it there were some serious bugs, but it's pretty stable now.
Didn't use the particle system except for some simple situations and it worked well there.
Reading the Blender-commit mailinglist I can see that heavy bug fixing is going in, especially at particle systems.
Blender 2.4x doesn't really have any benefits compared to 2.5 for me (exceptions: exportscripts like blendigo),
but maybe I'm a biased user since I tend to just fix occuring bugs by myself.


