I have an AR50. Armalite did an amazing job. The first time I brought it out, a friend of mine got a 2.5" group at 900 yards. Unbelievable.
The thing that impressed me most about the AR50 was the muzzle brake. You literally cannot feel any sharp recoil - just a slight push on your shoulder. I was told to expect a "30-06" type recoil. After the first rounds, I realized I could shoot this thing like a .22lr - a 40lb .22lr, that is.
Last time I bought ammo, it wasn't that bad in bulk (200+ rds) - not cheap, but I think mine were about 1.65$ per, for commercially reloaded lake city stuff. If you want to shoot it often, it's probably best to get a press (has to be large enough to handle .50 cases) and load yourself. It will still be expensive, but you'll save quite a bit. Also, since I can hit dinner plates at 1000yds with every round of bulk surplus component reloaded ammo, I am scared to think about how precise it would be with some nice handloads.
This is definitely the most fun firearm in my collection. I don't have enough time to devote to shooting as a hobby anymore (my IPSC equipment is collecting dust) but I will never get rid of this thing.
About the scopes - I bought a Tasco "Super Sniper" 20x 30mm scope. Cheesy name, but incredibly solid. It was fairly inexpensive, around 300$ if I remember correctly. It is ".50bmg safe" according to Tasco, and I believe it, as it has survived quite a few rounds through it without even needing to be re-zeroed.
The one thing you need to consider before you buy it is this: do you have a place to shoot it? It's pointless to shoot it at anything under 500yds - and really, that's much too close.
Now, if only my cars held their investment value as well as my guns.