The ones we use lose signal, are a pain when they are out of use because they need new uploads and if you can't use them and have to use the old method it causes chaos at the tills because they have to enter everything manually.
When they do work it is quicker than the old way.
Advantages: Make sure you buy a Pocket-PC type e.g. HP iPaq.
You can read eBooks on them in the .lit or .pdf format (just install either microsofts Reader http://www.microsoft.com/reader/downloads/pc.aspx or / and Adobe pocket reader http://www.adobe.com/products/acrobat/readerforppc.html)
Handy to keep in your bag for Internet/email purposes. Office software supported.
They even support TomTom mobile navigation software http://www.tomtom.com/products/category.php?ID=2
Great community support if you encounter problems http://www.pocketgpsworld.com/
Disadvantages:
You need to purchase additional memory cards as the built in memory capacity is not all that. (but SD slot-in cards are easy to install)