I am writing up my experiences as I found it very hard to get this information when I asked ;-) - but "management summary" - this will be a very positive product review.
What is the Huawei D100 Wireless Router?
It connect the 3 mobile phone broadband wireless network (which according to the 3 blurb covers almost all of the population) into your own local area Wifi network. Basically, two sets of wireless in one box - one to connect to "3" and the other to connect your own local devices. It also has an RJ45 ethernet connector so you can plug in a wired ethernet device.
How does the Huawei D100 Router work?
The Router has a USB slot in it into which you plug your 3 Dongle, and that provides a wide area network connection in very much the same way that the router we use to provide our regular broadband connection at home - except that the output is to mobile broadband rather that to cable (main home connection) or ISDN (our backup connection via another device).
The router provides a regular local area wifi connection (protected with a WEP key) and includes a DHCP server through which it can issue IP addresses. Initial configuration is to provide 101 IP addresses (192.168.1.100 to 192.168.1.200). It has its own IP address (default 192.168.1.1) and a browser interface there through which it can be configured. It forwards DNS requests too so that you can point your DNS at 192.168.1.1 and it will 'play' for you.
If you have a wired device you want to connect, you can plug this into the RJ45 port and use it via there. I have connected a hub, and been able to connect multiple devices. Devices which are manually configured with IP addresses on the same network (but not part of the DHCP range) ARE able to reach the wide area connection.
Some myths exploded
Some documentation talks about the router having a built in battery, but that is incorrect - it required power, and comes with a conventional external plug and adaptor. I suspect someone was overoptimistic in the product spec.
The device can be used (I'm doing it as I write this article) to connect a box running Linux; it is not limited to Vista, XP, 2000 and the most recent 2 versions of OSX. But beware - the rule is different for a dongle on its own (no D100 router) and it is just possible that something has to be initialised in the dongle that does require one of the specified operating systems at that point (I did my initial setup direct to a suitable laptop).
Words of caution
I understand that only certain specific models of dongle, which are some (but not all) of the ones that work on the 3 network, will operate correctly with the D100.
I have yet to establish whether or not the ethernet cable needs to be crossed - I can't at this stage provide easy advice as the hub I have connected is autosensing/
I have purchased a 5 Gb per month package which should do well for my needs, but I am very much aware that there is an excess charge beyond that which I believe to be quite steep, and I cannot at this stage give an easy comment on my usage.
Well ... here you go:
With a hub, here is the setup I had running yesterday evening in my hotel. There's a Mac OS X machine and a Linux system both connected at the same time; I'm running wired - primarily for testing - and both machines are online. I have switched the router's IP address to the same one we normally use back at base, and it's serving IP addresses in its class C address range. I also have my Linux eee PC baby connecting in wireless ...
a) The Router will let me get connect in my complete wired training network should I wish to do so and this gives me the option of providing delegates with a complete internet access at our machines in their own offices without the need for me to connect my kit behind their firewall
. Clearly, this facility has to be offered only on UK courses, and we must add "subject to availability of mobile broadband in the training room" - should be OK for the vast majority of sites; problems only at a few geographic locations, and where the building is in effect a Faraday cage - we get those occasionally!
b) The Router can provide a mobile backup internet service at both our HQ and Well House Manor, meaning that on any occasions that the cable connection is down (quite rare), we have an alternative service ... and we'll be able to retire the ISDN router / 'modem' that we currently have for the purpose
c) The router also provides an interconnection between the various wireless and wired devices connected to it ... so it's working (as I probably could have guessed) as a home hub too, rather than as a hotel or public access facility where the users are kept separate.
Other Solutions / links / details
3's own technical page
and a review from someone who read the "it has a battery" publicity
. An alternative product - the Dovado UMR
which is not tied to 3 and is available from Infoferenda
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articlesG913 - Well House Consultants - Hardware and Devices 
Setting up your MacBook Air as a mobile broadband router - (2013-07-07) 
Want to learn iPad and iPhone programming? Come along and learn with me for free. - (2012-01-28) 
Away to train - but still around by video for Melksham meetings - (2011-09-25) 
Change of balance, of attitude, and of work method - (2010-11-30) 
MacBook Air - hardware and system review - (2010-11-17) 
Web page to telephone calls / links using an iPhone - (2010-03-08) 
Camera to record where a picture was taken - (2009-05-18) 
Effect on server when memory runs out and swapping starts - (2009-02-26) 
Load Balancing - Hardware or Software? - (2009-01-15) 
What a shock - (2008-10-08) 
Latercomer to the technology - (2007-07-29) 
Hostnames spring forward - (2006-03-02)
Some other Articles
The Interview and The LiftJava Tag Libraries / how they work / Tomcat Deployment1234567890 ... coming up on Friday 13thThrough Snow and Flood to Linux and TomcatHuawei D100 Wireless Router - Mobile InternetMobile Internet - an alternative to hotel WiFiChoosing from an image with an image mapThe final step to being BritishAlso for the eyes of the catThings people say on the phone