Which NAS for Mac users?

You don't say how the server mac is sharing the HFS+ formatted drive. I would suspect it is AFP in which case the NAS drive format is largely irrelevant.

The NAS will store files on it's storage in whatever format is best for the OS of the NAS, and expose those files through network shares in a single or a variety of formats. The client machines mount the shares and should be unaware of the underlying storage (just as they will be unaware that it will likely be a RAID configuration of some sort)

Synology, QNAP, Netgear, Drobo and many many others all adopt this approach.

The major factor is getting a NAS supplier who fully supports AFP and does not just see it as an afterthought to be tagged on with a bog standard install of the AFP packages which may be out of date or have not been tested or tuned. The introduction of Lion saw a number of NAS devices with difficulties sharing AFP until they could be patched. Of course during this period the users could have switched to using CIFS shares and keep going.

I recently needed to replace an old NAS at home, and the cheapest option for my requirement was actually to build one and install FreeNAS on it.

You can add as many drives as your chassis will take, configure them to be RAID, mirrored, or independent, use UFS/ZFS on the partitions, and configure your shares as Windows, CIFS, or AFP with individual user accounts, groups, or getting permissions from an LDAP server.

Once you're set-up (which takes about 10 minutes from the CD) you're ready to go with a web interface which controls everything else.

While it's not HFS+, it is AFP capable out of the box so to speak, and works perfectly on my network with 2 Apple computers, and a Windows computer. It contains all of the resource fork information (meta data), and of course you can copy any existing data to it from anywhere else on the network.

Performance on a box like this is also going to be a lot better than most low to mid range NAS boxes too, which may be important with multiple users accessing large files. I backed up 580gb in just over an hour on a gigabit LAN connection.