I use it successfully for a subset of my networking needs (VPN server, VPN client, secondary wifi network, failover 3G/4G routing, security cameras, etc.). Of course the throughput is just 15 Mbps (VPN) to 50 Mbps (routing), but that's enough for these tasks.

Of course I can't route my primary internet connection (about 330-340 Mbps typical throughput) over it, but Asus RT-N56U does that just fine.

Fileserver is not an issue, because there's about 300 Mbps total USB bandwidth and the NIC is just 100 Mbps. IRQs are a bit of an issue, full 100 Mbit bandwidth generates 2k IRQ/s, USB disk will probably take it to 8k IRQ/s, consuming a significant portion of CPU (maybe about 20%?).

But if 10 MBps (80-90 Mbps throughput) fileserver performance is enough, RPi is just fine. About 3.5 MBps over ssh, like when using for example sshfs. For comparison, 3.5 MBps more than enough to saturate "54 Mbps" 11g for example, which can do 24 Mbps under ideal conditions (~3 MBps).