Does anyone know how these rifles compare as far as accuracy, durability, and quality.

With the Notable Exception of one particular company who shall remain nameless except for the initials (H.E.S.S.E.) who has a less than sterling track record for quality and who has just started to manufacture a .50 rifle/kit, any manufacturer of .50 rifles or uppers is a quality operation.  

Barrett, Serbu, McBros, State Arms, Armalite, Ferrett, etc. all put out a VERY high quality product.  

In order to handle a .50 round, any rifle you get will be durable--there is no such thing as a pencil barrel in a .50.  A few of the rifles are lighter than the others (the Serbu for example), but you can probably take any given rifle, use it as a prybar and still have no trouble with it when you go to shoot it later on.

That covers quality and durability.

As to accuracy--all of the .50s have something going for them by way of accuracy.  Any rifle you pick out will have won some kind of accuracy competetion somewhere--the State Arms, Armalites, McBros rifles are all regular winners, and the Barretts do pretty well also.
The Serbu rifles are also pretty damn accurate--even with a red dot sight on it .

So, in the end, it all boils down to what you want in a rifle--bolt or shell holder action, what weight you want, and what rifle you think looks the best.  Put good optics on it, and feed it good ammo, and you will be very happy with it.

AFARR