I used 2 LaCies for a while, but they both had a throughput of 10MB/s (the NAS with XP as OS) and 6MB/s (LaCie with Linux).
Then I switched to Synology DS408. Mine has 4x Seagate 1.5TB HDs, RAID 5, so I have around 4TB of space.
The network throughput maxes out at around 60MB/s(!). But this might be due to my not-so-good switch. It's all on a Gbps-Network.
I used it only with Mac OS X (iMac, MBP, MBA, MB) with AFP. I haven't tested performance with SMB or NFS, but should be as fast as AFP (probably even faster).
One thing, which really convinced me of Synology, was their support. Since the Seagate 1.5TB HDs have some problems (make sure you buy those with Firmware >=SD1A), I had a lot of issues at the beginning and thought that it's a problem with the NAS. I even thought I lost data. When I contacted Synology, they offered to log-on on to the NAS and try recovery, local check and everything - for free. And in the end, they found the problem with the Seagate HDs, proposed the solution and I am now even more happy then before.
And no, I'm not working at Synology...