Using Debian Squeeze (Linux 2.6.32 and a custom 3.5.2 kernel both) I was able to install Reason 5 without too much difficulty. However, there are a few anomalies that keep the app from a silver medal in my case...
There is a lot of graphical noise in Gnome 2.30.2 (using Metacity, no compositing) and most of the application toolbars and dialog boxen are covered in black, making the configuration pane, editing toolbar, and some of the toolkit (F8 screen) unusable. Because I have experience running Reason in Windows, I am able to get by on rote experience but would be lost if this was my first time running the app.
Out of the box, Reason emulates DirectX Audio as well as I would expect Windows to, without dropouts or skips at the default 92ms. I was able to reduce the buffer and get 65ms output without skips.
The menus do not work by mouse, which is a common issue for those running Reason over Wine. The work-around is to press ALT, then down-arrow to expand the File menu, and left-right arrows traverse the Edit, View &c menus. Also, I have not yet tested MIDI controls, having just used the on-screen keyboard (F4) for testing.
All said, however, I am able to work with Reason adequately to not have to dual-boot Windows at this time, and that's great.