To re-iterate what others have already said, from my experience of the AR20:
Feature-wise they punch above their weight and offer a lot on paper. Strikers are light and have a short stroke which will make for a forgiving, fast lock time. Triggers are only pseudo-2-stage (so mechanically inferior to the S400 unit and the like) but are very crisp, light had usually give good feel.
The action and stock are ambi and the stock offers plenty of adjustment at the cheekpiece and pistol grip. The 300bar fill is tempting on paper (as are the twin regs) however it appears that in practice they don't function so well.
Looking purely at features (multi -adjustability, twin regs, 300bar fill, arguably decent trigger) it's hard to argue that there's much that can challenge the AR20 if buying new.
Of course there's no such thing as a free lunch and the AR20 is no exception. What's gained in headline features is lost in build quality and longevity - these guns are not built to stand up to the rigors of time; and in terms of construction are grossly inferior to alternatives such as the Air Arms MPR and EV2.
The action is a die-cast sandwich assembly that splits down a centre-line, and is held together by nasty self-tapping alloy screws (the same as Umarex use in the CO2 pistols, for those who've had one apart). I suspect these would only last so many fitting/removal cycles before the threads are gone; and as such require a certain amount of care.
Alloy castings are not restricted to the action block and there's lots of plastic used on the gun. It's clear from the fit and finish the quality of the rifle is a far cry from that of it's competitors.
To further put the boot in, IIRC (and I'm happy to be corrected) the regs offer no adjustment as to output pressure, there's no way of testing the pressure on the gun and to the best of my knowledge nobody has an appropriate reg tester for these guns (although one could be made).
Over time the disc springs in the regs fatigue and lose their "spring" a little; reducing muzzle energy. On a normal reg the remedy would just be a case of tweaking the preload screw and using a tester to ensure that output pressure is correct.. this isn't possible with the AR unit and the official line is to replace the reg (not sure on cost but regardless not a great proposition every 2-3yrs).
Unfortunately the after-sales support from Armex might favourably be described as "poor to non-existant" - they seem very much a company concerned only with "shifting units" and are far less bothered by offering support for said products once they have the customer's money. I'm not sure to what extent these rifles are supported by enthusiasts.
Finally as regards the new price; "official" imports through Armex cost dealers a lot more than grey imports from other sources (make of that what you will). IME this is usually the cause of price discrepencies. Most dealers will have an account with Armex so will naturally turn to them for supply; probably before checking other sources.
As an aside if you must have one the Euro is particularly weak against the pound at the moment; meaning you could import one for around the price of many used offerings.
Ultimately I wouldn't touch one of these with a barge pole since they're not made to last and I can quite easily see them become worthless in future years; due to their cheap build, lack of after-sales support and perpetual reliance on the replacement of significant parts (which will most-likely become hard to source).
Personally I'd be looking at a used MPR or EV2. The MPR should only need minimal replacement parts to keep it working perfectly for many years (seals and exhaust valves; unless the valve is replaced from the off with one that's made of something harder than toffee). On top of seals the EV2's reg will need some attention periodically, however these are well-catered for and should generally just be a case of a strip, clean, new seals and a setup - rather than a complete replacement unit.