Stefan:
I recently saw that RandomControl now offers also a learning edition of Arion 2.
My questions, if you don't mind, are does Arion's license allow you to install it on two machines,
because i would like to set-up my scenes on my iMac (Win XP) and later render them on my Notebook (Win 7), which outperforms GPU wise my iMac.

I don't know exactly their pricing & licensing,better to contact them to clarify your doubts
anyway Learning Edition allow you to use all GPU's of your system
Network rendering NOT allowed
max resolution:1920x1080
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Stefan:
Can the preview rendering mode, which i saw on a youtube demo, be turned of, like working in Maxwell Studio, prior rendering

Arion is a real-time application,this mean that just after you loaded the scene render starts immediately.
You can't quit the render,save the framebuffer and resume later for more computation(like Maxwell and Fryrender)
The preview mode is what you get,just choose output size and format so it's a WYSIWYG application
Setting camera focus and position,assign materials,all done in the preview window

You can choose which devices will be involved for computation,which videocards and how many Cpu's cores
Being a CUDA-BASED application you must have an NVIDIA card,if you have an ATI you can use only CPU(than you loose all speed benefits fo hybrid calculation)
They suggest to use two videocards:one for computation,other for UI and navigation.That's the main thing!
Normally you got just one videocard,this one must handle UI+navigation and also all computation that Arion require
this mean a general slowdown or a freeze in all UI operations(open a new window,navigate on Internet while Arion working,change a material,moving the camera,etc)
you can switch Arion in PAUSE and do all you need,or change your scene settings,but sometime changes you made are not updated in the preview window
this depends by your videocard and the amount of video-memory(I'm experimenting this with my GTX 460 -1Gb video RAM)
if you plan to manage heavy scenes or render at high resolution you must have a videocard with at least 2Gb video RAM and the highest number of CUDA-CORES
be careful when watching demos on Youtube:you see Arion performing in real-time but they don't tell what hardware they are using...
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Stefan:And last but not least does Arion 2 comes with a (good) material library?

Materal library comes in 12 sections:

AUTOMOTIVE:6 carpaint materials
CLOTH:2 satin materials +3velvet
DIFFUSE: 7 colors
EMITTERS:14 emitters (D50-D65-tungsten-studiolamp-flash-white-candle) IES are supported
FROSTED:5 variations of frosted and smoked glass
GEMSTONES:11 different gems(amethyst,aquamarine.topaz,etc.)
LAMBERTIAN:7 colors
LIQUID:6 (bordeaux wine,water,oil,ice,etc)
METAL:5(aluminium,copper,gold,mirror,silver)
PLASTIC:19(rubber,plastic,marble)
SSS:12 (beewax,rigatone pasta,acrylic etc.)
TRANSPARENT:12(dispersion diamond,transparent plastic,smoked glass,etc.)

They have improved materials behaviour from Fryrender to Arion and i can say that you got top quality
Metals are not the best area (it's the most difficult stuff to replicate in all render engines,unbiased or not)
Architecture and interiors are best fields for Arion,sun also works good for outside scenes
Best of all if you are a Max OR XSI user,so you can manage all setting inside them using LIVE plug-ins
i get benefits using my old FRY-C4D plug-in to manage the scene than export all and open in ARION
i think that is slightly difficult have the standalone version and manage all settings inside arion after importing as OBJ
in that case you can't have full control like with the plug-in..no doubts you must have a strong videocard

hope this help little more

M