To me, it has been worth every penny just because it's something completely new to me. I came from being a long time follower of another franchise and was always hesitant about arma2 due to controls and then i kept seeing talk of arma3 and decided to hold off on arma until it was available. The SP in BF3... I won't even get started on my rant with them but you aren't going to be walking into the same game. This is a thinker game with tactics and teamwork and is a sandbox. If you want to wait for the alpha lite invites to be activated, i personally will send you one of my alpha lite invites and you can judge for yourself.
The big plus, is you can play how you want, and you can make your own missions with AI and everything done the way you want. If you want a flexible environment with mods and custom maps and modes, i think you'll be satisfied. If you're expecting to walk in and replace MP of BF3 with this, it won't have the same pace that you are accustomed to.
As for system specs, i'm not sure you might need to look around and see what most are running. Sorry i just don't have enough experience with the series to know how it performs on various hardware but the info is out there.