Improving a weak wireless connection is a tricky proposition for many Wi-Fi users, mainly because they lack the knowledge of what is causing the poor connection. Many manufacturers take advantage of this lack of customer knowledge and just put increasingly larger speed numbers on product packaging, leaving the purchaser to imagine that a bigger number means better performance. Maximum speed, of course, doesn't necessarily yield better performance, especially when your wireless problems are caused by interference from an abundance of neighboring networks.
To hField's credit, they provide good advice to potential buyers on both their website and the User Manual that comes with the product. The advice boils down to the fact that the Wi-Fire can make a weak signal stronger, but may not be able to provide connection to WLANs that are currently beyond the range of your current adapter. I think hField is taking the right approach with the advice, since, as a small company selling the product directly, they can't afford to have a lot of product returns.
If you are currently using a notebook with built-in WLAN, you may have an experience similar to mine and not see a significant improvement. Part of this is the superior antenna orientation provided by having the antennas embedded in the vertical notebook screen, which is a big improvement over the low, horizontal antenna orientation afforded by a plug-in Cardbus card. The Wi-Fire, with its movable antenna, provides a definite advantage over both built-in and Cardbus WLAN adapters because it lets you position the antenna for maximum signal independent of your computer's orientation.
But also, as in my case, the chipset in your current wireless adapter can also make a big difference. Adapters using Atheros Super G and Airgo True MIMO chipsets already have excellent receive sensitivity and can provide throughput vs. range performance equal to or perhaps better than the Wi-Fire.
In the end, the Wi-Fire puts the methods that have the greatest chance of success for wireless signal performance improvement - a higher-gain antenna and the ability to flexibly position it - in a handy package that can be used for both notebooks and desktops. Whether it helps or not, however, depends on what you are starting with.