If your an occasional user of digital voice recorders, this is a great product and you will likely be very happy with it. I "upgraded" my VN-4100PC to this unit because of the lack of Vista 64 support on the VN-4100 PC, I still was able to offload audio using VMWare with a XP image.
The device is a lot larger in terms of audio storage than my VN-4100 PC, I went from 24hrs to 70hrs with HQ mode (highest quality non-stero mode). On lower quality you can get 250+ hours, but I find HQ is best because you are better able to understand what you are recording.
I use the devices two ways, one way is very difficult with this device and is the primary way I use it. The other way, the device works perfectly and well suited for this task.
I will frequently record phone calls with a TP7 telephone recorder (very cheap mic that records both sides of the conversation). For this task, the WS-400S is excellent, the recording time and quality works perfectly. Since the device is now a mass storage device, support for any OS is practically guaranteed.
The second, more common use is taking notes. I use David Allen's "Getting Things Done" process as well as other systems and my own for day to day productivity. One part of GTD I have adopted is taking note down immediately. I use a digital voice recorder for this and carry it just about everywhere. I will then dump the notes into my system, here is where I have a lot of problems. With my VN-4100 PC I would put the device on "HOLD" which pretty much locks the keyboard, but with the WS-400S, it turns the device off, the downside of this is it takes 1-2 seconds for the device to wake up, compared to the instant wake up on the VN-4100PC. So when you take it off hold and hit record (I frequently do this without even looking at the device since I record 5-30 messages daily) the record button frequently will do nothing, you have to remember to wait 2 seconds or so before even attempting to press record. This has caused me numerous lost recordings and 'redos'. For daily use, this has become much more problematic than lack of Vista 64 support has been. Support claims this is par for the course and there is nothing that can be done. I am waiting to hear if the WS500 series has the same problem, if they do I may want to look at Sony recorders. USB functionality is key for me as I frequently record phone calls and my son talking for my blog. Also as a mass storage device, it is actually easier to use the computer to record 10-50 messages a time to put into my "system".
I would give the device a 2 because this slowdown is such a problem for me, but other than that, the device is excellent. It is just slow all around because of the new computer system on it. If your a daily user like me, I would stay away from this device. Unfortunately I already gave away my VN-4100PC so I will be looking for another device and send this one back.