I have TS-459 Pro+ and Lion (Works)
I'll try to give you the straight scoop on the Qnap...
I have the Qnap TS-459 Pro+. I use it to support GNU/Linux boxes and Mac boxes. Currently, it's working well and I think it's a good product.
However, I can confirm the earlier poster's comments about (previous) trouble with Wi-Fi. There was a good amount of time where TimeMachine backups over Wi-Fi to the Qnap was broken. I use two Apple Airport Extremes for my Wi-Fi and for quite a while the Qnap would not work on the latest (at that time) firmware. I had to downgrade Airport Extreme firmware by one version (luckily easy to do) to work around the issue. That is no longer an issue. I'm on the latest firmware on the Qnap and the Airport Extremes and Time Machine backups over Wi-Fi are fine.
I can also confirm the AFP issues. Note, this was NOT a Qnap issue and affected several major NAS vendors. The problem is that the NAS vendors typically don't have official AFP protocol stacks from Apple. They're using the Open Source product Netatalk. For right or wrong, recently Netatalk became irritated with all these NAS vendors making stacks of cash with their project and not contributing back. There was a bit of a stand-off, it seemed, but that has apparently been resolved. For a while the Netatalk developers weren't releasing the latest version of their product that supported the necessary AFP specifications, particularly used with Lion. I'm guessing Qnap (and the other vendors) must have eventually thrown some money at them or whatever. Anyway, Netatalk was updated in the Qnap firmware quite a while back and it's been fine for quite some time now (several months).
Regarding Lion, I also had to wait about a week or so to ensure it would work with Lion. There were some hacks folks had figured out in the Qnap forums to work with Lion on release, but I didn't want to screw with that and decided to wait for the official update. So, yeah, that was a week or so behind the Lion release, so maybe that's important to you. Again, it's working great with Lion now.
So, as it is now, Time Machine backups and restores are working with the Qnap. I am not aware of any outstanding issues at this time. Many of the NAS vendors are running GNU/Linux and Open Source applications behind the scenes to provide their functionality. The nature of this means that they don't have any inside link to Apple or anything to get the specs ahead of time. They often have to figure out the protocols on their own and then the NAS vendors just grab a stable release. So, the nature of this means you should probably expect a bit of delay from time to time if you use a product like this. I don't think that is a Qnap-specific problem.
Another "issue" I've seen has to do with sharing my music on my local network. The Qnap has some features that I really like for saving energy, basically putting it to sleep when not in use. It was very common that iTunes would complain when I synced my iPhone or tried to play some music from the AppleTV if I tried to do so when everything was asleep. I don't think that iTunes is patient enough for the Qnap to wake up (not that it takes a long time, but there is a delay when you first wake it from sleep) and iTunes would think there was a problem. To work around this, you can just tell iTunes to play a song and skip over a few. That seemed to do the trick and it would behave, but I must admit that I eventually got tired of doing that and ended up copying my music to the local drives on the Macs so that everything is instantaneous. It's silly to have the music installed everywhere, so I thought that was worth mentioning (and of course, you want to tell Time Machine not to backup the music if you already put it on the Qnap).
So, those are the mild issues I've had. As far as the product, it rocks. I love it and if you're a geek, you probably will too. If you're not a geek, you should probably just go for a Time Capsule or something. But yeah, I like being able to get into the shell on the NAS and geek around in there. I use it as my subversion repository. I like the torrent feature so that I can have it deal with my GNU/Linux downloads or whatever. I dig that it can talk to just about ANYTHING. It supports tons of network protocols. It has tons of cool features due to the GNU/Linux it's running. And it has a decent community for adding stuff and tweaking if you're into that. You can also do cool things like remote, encrypted backups if you have another buddy with a Qnap, which might be nice if there's a fire or burglary or whatever. And it does the whole camera surveillance thing, which is pretty sweet if you're sort of paranoid. haha Plus, being a proper NAS, you can hot-swap drives when they go bad, which is probably the most important feature when comparing it to something like the Time Capsule.
But again, if you're not going to geek out with it, then a Qnap is probably overkill and a Time Capsule will probably be a good amount cheaper and maybe smoother if you're really just focused on Time Machine.
But if you want to geek out, if you're into GNU/Linux, and if you're on a network where you want the NAS to work with several different devices of various platforms (meaning, differing operating systems), then the Qnap is a pretty awesome choice.
Oh, and I am not using the iSCSI feature. So, I can't comment on the previous poster's issue there, but I can say you shouldn't be required to use iSCSI if there is no need for it. I'm not using iSCSI and I'm connecting to quite a few different Apple products without issue.
I hope this helps.
