How well does an ATI mobility radeon hd 5650 work?

According to this post on the ArchLinux forums, the proprietary flgrx driver supports this card.

It should therefore work with Unity.

I have the same GPU (Intel/ATI 5650, switchable graphics). Yours is similar probably.

AMD has a driver that supports switchable graphics:

http://support.amd.com/us/gpudownload/linux/Pages/radeon_linux.aspx (11.5 as of this edit)

I highly recommend using the instructions for your version of Ubuntu here:

http://wiki.cchtml.com/index.php/Ubuntu

I used the instructions for 11.04 Natty and Catalyst 11.5 and the installation was smooth and successful. I suggest reading the page thoroughly before proceeding so that you know how to recover in case something doesn't go well.