We've been using a LeftHand iSCSI SAN for over two years now. We have 3 XServes that access the SAN, as well as about 10 Linux servers, some of which form a Xen cluster. The SAN consists of a pair of HP DL320s in a mirrored failover configuration.
On OS X we use GlobalSAN as the iSCSI initiator and it works extremely well. We only had a few crashes with earlier versions but the latest builds have been rock solid. They handle the Virtual IP handoff of the SAN very well in a failover scenario.
The LefHand units we have are configured in RAID 5, I don't know if the newer ones support RAID 6 or not but quite frankly I don't think we would use it even if they did. By having two, the setup effectively becomes a RAID 1+5.
The LeftHand SAN/iQ software supports asynchronous replication as well as snapshots, and does both quite well. They also have a neat feature in that you can set up a virtual SAN appliance in a VMWare host and use it to migrate data between SANs without purchasing any special additional hardware.
SAN/iQ 8 also supports forking of snapshots, and is a huge improvement< over the previous version. My one big qualm with it is that it still has pretty crappy scripting support and we had to write our own Python wrapper over their scripting interface to make it work for our purposes.
The other downside is that the units are pretty much a black box. Other than using their management console or the scripting interface you can't really have any idea as to what is going on inside. There's no way to SSH in to the unit unless you have the special keys that the techs do.
Also, if you expect to get technical support (which, by the way, is excellent) you should probably have a Windows machine handy with Putty available as it seems to be the only thing the first tier support guys are familiar with. This may change now that HP has acquired them. That being said, even the first tier guys are very knowledgeable about their products and have always been able to give me good support. For bigger problems I've been able to work my way all the way up to the engineers who work on the product.
Overall the experience has been extremely positive, and our entire office network uses this SAN for pretty much everything that involves storing data. It integrates very well with our backup and virtualization strategy. I don't know what your intended use case is, but for something like our scenario I would highly recommend the LeftHand products.