While the software on the Fritzbox for 3G connection sharing over Wi-Fi is still an early beta, I have recently also tested the Huawei D100, a box dedicated for 3G connection sharing. So far, I've seen it advertised by 3 in the UK (with contract only) or by 3 in Austria, here unlocked for 99 euros. Needless to say I preferred the Austrian variant and went into a shop while in Austria recently to buy one. I have now used it for several days with a E220 3G USB stick in the Mobilkom A1network and I have to say I am very impressed by stability of both the Wi-Fi and the 3G connection and the ease of use of the box.
Once power is switched on and the power on button has been pressed, the box automatically attaches to the 3G network and establishes an Internet connection. What I've found a bit odd is that the power button has to be pressed for the box to start. This is a bit inconvenient after a power outage for example. Should the 3G connection drop while being online, the box is smart enough to figure out something is wrong and automatically re-establishes the connection within a couple of seconds. It happened two or three times in the course of a couple of days but it's hard to say if that was due to the D100, the 3G stick or the network.
The Austrian D100 comes preconfigured for all Austrian networks, but it's also possible to add configurations for SIM cards of other operators and countries. The page for this is a bit hidden but one found, it's pretty much straight forward.
While most people will probably use it for Internet connection sharing, Dean Bubley recently had a number of other interesting ideas of how to use what he calls 'the dongle dock'. Have a look here, especially the last paragraph. Now I need to get a femto to test his idea :-)