Network storage device replacement for Airport Time Capsule

I would be interested to know if anyone has replaced their Airport Time Capsule with some form of network attached storage device, and how successful this has been?

I currently have a 3TB Time capsule, that is full and frequently produces errors, such that I need to start a backup afresh. I am worried that one day the drive will die. We have 1 desktop iMac and 5 MacBooks that need backing up, so we definitely need a network device. We don't tend to do any streaming from central storage, so all I am really looking for is a reliable fast ethernet connected hard drive that will provide 6 Time Machine backups. I guess I would want 4TB minimum.

Thanks

Peter