The bag needs loops in the saddle to fit well such as found on Brooks models. Or you can buy the quick-release rack from Carradice that supplies the proper mount points for all bikes. The Sport-size Carradice rack is plenty large enough to hold the bag. The rack is actually suitable for the next larger sized bag as well.

You don't absolutely need the rack for a smaller bag like this, but you might find the bag hangs too close to your legs when strapped to the seat post directly. The rack also helps stabilize the bag from swaying side-to-side as you climb.

You do not need a bag support if you:
-always park your bike indoors
-have a saddle with bag loops in a high position
If you don't meet these criteria carradice makes a couple of excellent options, quick release bagman support and quick release sqr support.
I use this bag on a roadbike for commuting; I find it exceedingly suitable.

This depends on the height of your saddle relative to the wheel size or fender. With my seat at 73cm (center of bottom bracket to top of seat) and 700c wheels no support is needed. There is a strap to secure the bag to the seat post. However on my girlfriend's bike with seat height 60cm and 700c wheels, a support is needed.

For road bikes this bag is great if you want to carry more than just your lock and tools.

I use the Bagman sport QR on my road bike with my Barley.

Bag support not essential. Very suitable for roadbike.

Yes suitable for a road bike. I have a Trek 1.2. The bag does need a Carradice support, which I bought. The only problem I have had with this is that the fixing to the seat started to come undone. So will need to check often whilst riding.

If you want to use the Barley on a Road bike (yes, it works fine), then I would suggest buying the Carradice Bagman Quick Release Saddle Bag supoort. http://www.wiggle.co.uk/p/cycle/7/Carradice_Bagman_Sport_Quick_Release_Saddle_Bag_Support/5360042588/
If you have a Brooks type saddle with bag loops you will be fine, else you will need to rig something else up.

I wanted the quick release model but it was out of stock so I purchased the the standard model at less than half the price. A bit more fiddling to fit but it works great. Stops the bag from swinging into my legs when on the move and a quality product. It also holds the bag in a better position when you hang a tail light on the strap. I used the bag without one but I'm glad I bought the support hope this helps in your decision.

this bag will work on most road bikes, but attachment depends on the availability of bag loops on the saddle. many road bike saddles do not have these loops. also, depending on the geometry of your road bike, the bag may hang at an awkward angle. a bag support would probably improve the positioning of the bag on any bike, but is often not needed. i think it's a matter of trial and error.

Those fixing pieces are "Cyclo" bag loops - searching Wiggle, I couldn't find a listing for those, or for the clamp Carradice supply themselves ( a sort of block type thing that clamps to the saddle rails - this has two slots in it for the bag's straps).

A google search should turn up a supplier for the Cyclo bag loops - bear in mind that supply of these seems to fluctuate, so it could be an idea to buy them while they're available!