Not sure about those negative reviews - must have been a bad batch?
It does what it says it does, basically. You'll know when it's plugged in - the slider that extends the USB plug glows bright orange.
The only slight snag I found was that the catch on the slider is sensitive, so when you insert this drive, hold it by the sides, not the faces. Other than that, I can't fault it, and certainly not when it's on offer at £18 or less!
This is a U3 USB Drive. This means that it will appear as two drives in Windows Explorer (2000, XP, Vista onwards) - one read-only "CD ROM", and the other as a normal removable storage drive. The read only partition contains the U3 software, which by default is set to auto-run when you insert the drive. This places a moveable icon on the desktop which is used to access the various functions and programs. This can still work, even if you have disabled the auto-run feature in Windows, by installing a very small program in your startup sequence that detects U3 devices. This is what I have done, and it works well.
If the U3 functionality is not for you, and you just want a 4GB USB drive, then it can be completely removed, restoring the drive to a single partition. However, this removes the ability to (easily) encrypt the drive contents (128bit AES), and make use of an excellent selection of portable programs, including Firefox U3, Skype, Outlook sync, and many others.
A U3 program runs entirely from the USB drive, and installs nothing on the PC it is plugged in to. This is very useful if you find yourself using a public computer. A good example is the built-in Outlook Sync App, which opens as "look-a-like" Outlook, and allows you to access your email on any PC with an internet connection, whilst retaining privacy. Emails that are downloaded whilst on a public PC will be syncronised with your "home" PC when you next plug it in - those emails are also not stored on the public PC.
For the ultra paranoid, a U3 version of the popular iPig VPN client is available. This allows you to encrypt your internet access when using a public PC - in a cyber cafe for eg. This is only really of use if you have reason not to trust the owner of the public network you are using!
The drive operates at speeds you would expect from a USB 2.0 device, ie it's quick. Works fine in older USB 1 ports too, although obviously slower!
For anyone wanting more space - 8GB and 16GB versions are/will be available at some point. Have a look on the SanDisk website if you're interested in the details.
Or, you can save by going smaller. The 2GB version is £10 at the time of writing.
In summary, this is a brilliant buy. Highly recommended.