I guess experiences vary. I haven't had a 3D-related crash on HD 4550 in over a month.
The open drivers do require a recent kernel and the mesa stack built from source (I believe there are packages for Ubuntu). They provide good OpenGL 1.5 acceleration, and GLSL is coming soon. Not as fast as fglrx yet. Tear-free XVideo. KDE4 effects work great.
There is only very basic powersaving now (and none if you use KMS). There is no decode acceleration, and likely won't be any for at least half a year, as it will be built on top of the Gallium driver, which is nowhere near ready yet.
This is the state of the open-source stack. I've never used fglrx, so I'll let other people answer that.