Note: all of this is my interpretation of reading A3's website
It's excellent value if all you want is simply *G and nothing else. It will be interesting to see how it all goes when A3 is merged with OA, of course (and the resultant airline is assumed to still be part of the Star Alliance). OA's FFP has two status tiers as well, but the qualification criteria are a bit higher.
Some people in the BD program might find A3 more accommodating compared to the inevitable LH program.
A3 is less generous crediting miles for particular *A partners. For example, UA First credits 300% miles, but US First only credits 150% (very different to BD and LH). SQ only credits 160% for Suites and 150% for First, but everyone kind of knows that SQ is pretty stingy. Only some other select airlines give 300% for First class.
As for the rest of the classes, most look pretty standard and in some cases there are less non-credit classes compared to BD or LH. All mile earns are subject to a minimum mileage as well.
As for burning, the awards chart for Star Alliance carriers looks a little steep compared to BD. According to A3's website (or at least how I read it), awards are redeemed flight by flight, e.g. if you fly from LHR to SYD and you need two flights, you need to work out the mileage required for each flight separately, then add the two costs together. (A flight in this case is tracked by flight number). As far as I can see, no cash+points awards either.
Redeeming for A3 flights seems considerably easier and (on first sight) cheaper.
As for elite benefits, A3*G get a couple of benefits when you depart from ATH, but otherwise all other benefits are standard *G benefits.
