When I got my AT&T 3G Microcell, I was surprised to learn that it was completely unusable because it couldn't get a clear GPS signal at my home. The broadband connection worked fine and my phone saw the Microcell just fine, but without the GPS location, the device won't let you make any calls. I tried placing the Microcell in various locations, including smack in the center of a big bay window, but no luck.
So I ordered this antenna and it made all the difference. It was quick to plug in an route outdoors; the wire is this enough that I was able to route it through a nearby window. And it works great. Within a minute or so, the GPS was working and so was my Microcell.
My only negative about it would be that the connector is pretty flimsy and actually broke one day when the wire got tugged by accident when a friend's dog happened to hit it with a paw when looking out the window. It still works, but the connector basically broke. (Fwiw, I this was so cheap and worked so well for like 18 months that I just ordered a replacement.)
AT&T really should include this with the Microsell. The fact that they don't just tells me that they're out to rip off customers, because my Microcell was completely useless without it. But with it, it works great.