I won't re-hash what others have said, but the drive I/O performance (or lack thereof) is a real problem - especially with Time Machine.
I have this device hard-wired into my router (Gb connection) and my MacBook Pro is also hard-wired into the same router. My initial Time Machine backup went with reasonable speed, but subsequent, incremental updates to the drive started dragging. I found that if I reboot the WD MyBook Live, it works better for a period of time, but then slows down. With wired, Gb networking end-to-end the Time Machine performance should be on par with my 800Mb Firewire portable drive, but it's not even close. Even just copying files via the Mac Finder over the (wired) network to the drive is agonizingly slow.
That said, however, their iPhone app and web-based access tools are some of the best I've seen from this product category, so I will keep the device and see if things improve with a future software update; hence the 3 stars.
Finally, I was hoping to use the DLNA server to stream high-resolution FLAC files to my network receiver (Denon DNP-720AE)... don't even think about doing that. The WD MyBook Live comes with an old version of Twonky Media Server that is incredibly buggy. Really, one of the most frustrating experiences I've had with a gadget in recent memory. There are instructions on the web to allow you to hack your drive and update the Twonky server, but I just don't have the time for that - especially since I have little faith that an updated version of the atrocious (and appropriately named) Twonky Media Server will be any better.
UPDATE: Well, two weeks later... I decided to try manually upgrading the Twonky Media Server to 7.0.9 Special (version specific to NAS devices). If you Google around, you will find step-by-step directions on how to do this; if you are not afraid to ssh into the device and manually download and install software on the embedded Linux system, you will find it is time well spent. Twonky version 7.X seems to function correctly and is robust; I am happily streaming 24/96 FLAC files to my Denon, using the Android App BubbleUPnP as the control point. Works great.
However, the Time Machine issue remains a show-stopper. I really *do* need to periodically reboot the drive or Time Machine incremental updates never finish. That is, it takes more than an hour to backup a single GB of changes (over a wired Gb network) and then it starts all over again. Rebooting the WD MyBook Live and restarting the Time Machine backup moves very quickly through the backup process. If I don't see a software update from WD in the next couple of weeks, I will have to move Time Machine backups to another device. Of course, it's possible that the issue is with Mac OS X (mountain lion); time will tell, I guess, but I find it odd that I can restart the WD and it moves fast, again. Restarting Mac OS X and restarting Time Machine does NOT seem to speed things up, on its own.
UPDATE 2 (Sep 2012): Well, more time has passed and WD has released a F/W update. From what I can tell, the update did nothing to correct the problems I describe above. However, I finally decided to SSH back into the device and see if I could figure out things on my own. There is a daemon located in /usr/local/mediacrawler that seems to be the culprit. I removed the executable bit and restarted the device. File I/O performance with Time Machine and AFP file copies has drastically improved. The mediacrawlerd program was absolutely hogging the device's processor to the point of seriously degrading file I/O performance. FWIW, I posted my discovery on the WD support forum but have heard nothing back. Who knows what mediacrawler is supposed to do, but for my use cases it seems nothing useful. Time Machine backups and the Twonky Media Server run like champs! It's inexcusable that WD isn't aware of this issue, but in scanning the WD support forums they don't seem too responsive to customer complaints. I don't like writing bad reviews, but unless you are willing to SSH into the drive and upgrade/manage the embedded software, you should avoid this NAS device.
UPDATE 3 (Oct 2012): WD MyBook Live has an embedded program called mediacrawler. I am not sure what mediacrawler does (I think it is indexing files for use by the remote access tools), but it severely degrades file I/O performance.
I started noticing that TimeMachine backups were never completing - even with very small changes to my local HDD. Time Machine would sit forever trying to complete a 60MB (yeah, MEGAbyte) backup. And this is running Gb ethernet between the MacBook and the WD MyBookLive (i.e. not wireless). When I stopped Time Machine, I noticed that the HDD activity light on the MyBook Live would keep flicker forever.
I SSH'd into the box and ran 'top'. Lo and behold, mediacrawler is using a huge amount of CPU and never stops running. Sometimes it would help things to reboot the system - for a while - but eventually the drive gets back into the same state. So, I've updated to the latest version of the firmware. Nothing improved with mediadcrawlerd. While mediacrawler *is* using a lot of CPU, the much larger problem is the constant disk contention. After I removed the exec bit from mediacrawerd and restarted, Time Machine is (again) running like a champ. I am watching the Time Machine window report only five hourse remaining for 250 GB of 510 GB. Not bad.
With mediacrawler disabled, the Time Machine performance is very acceptable. However, it's clear that no one at WD is testing Time Machine backups for performance and while I have posted this finding on the WD support forum, no one seems to care.
UPDATE 4 (May 2013): WD MyBook Live STILL has an embedded program called mediacrawler, after the recent update. You STILL need to SSH into the box to disable that daemon. Again, I have no idea what that program does, but disabling it does nothing to change the functionality of WD2go or Twonky Media Server, as far as I can tell.
Since writing the initial review, I have changed out both my Windows laptop and my MacBook Pro (to a Lenovo Yoga w/ Windows 8 and MacBook Pro Retina). The Windows 8 file access performance is quite a bit faster than the Mac, anecdotally speaking - I haven't taken the time to formally measure, but syncing the same set of files between the machines is a night and day difference.
The latest firmware update does seem to work better with Mac OS - I have the belief that the performance problems are a combination of Mac OS's poor handling of NAS shares with Time Machine and a poor Apple File Protocol handler on the WD MyBook Live (afpd). I say this because even scanning a directory or copying files from my Mac (over Gb ethernet) is slower than it should be; especially compared to my Windows laptop.
That said, with the new update, my Time Machine backups are at least finishing in a reasonable amount of time and I can support backups from two Mac Books at the same time.
Still 3 stars - all of this was way too much work for most folks to deal with and I cannot recommend this drive to others. Spend more money and buy a commercial NAS alternative, like QNap or Synology.