As I mentioned, I had no difficulty getting a signal during my trip. The MiFi hooks into the network of a local carrier, which in my case was Vodafone Spain. Of course, since you're using a cellular network, your experience will vary by location. You should be fine in urban areas, but signals can peter out in rural areas, underground, or in the interiors of large buildings.
The promised battery life is 4 hours with a single charge and 40 hours of standby time. In practice, I was satisfied with the MiFi's power, though I never quite received a full 4 hours. I was using the heck out of the thing, and I ended up charging it frequently. Fortunately, you can continue to use the MiFi while it's charging, and the extra battery offered more juice in a pinch. Of course, to replace the battery you will need to terminate your connection and restart it.
Rates and coverage
The MiFi works in 39 countries and U.S. territories. That may seem like a comparatively small number, but it includes most European nations, China, Brazil, Argentina, Mexico, and Japan. Check out XCom's site for the full list. On the downside, promised speeds vary widely by location. You're promised the aforementioned 7.2Mbps in most places, but in Russia, for example, the company says you'll hit only 1.8Mbps. It all depends on the capabilities of the local carrier and whether you have access to a 3G HSPA/UMTS network.
It's also unfortunate that you'll need a different MiFi for each country on your itinerary. They'll be labeled clearly, but carrying extra devices is never ideal. As it stands now, you could need up to six devices for a standard tour around Western Europe. And, even worse, each device will cost you.
As you might expect, using the MiFi won't come cheap, particularly with the extra fees involved. The initial rental fee is $17.95 for each day of your loan period (it doesn't matter whether you've used the MiFi each day or not). Any additional devices for other countries will then be an extra $9 per day. Fortunately, you get unlimited data use, but it would be nice if XCom adjusted the fee depending on your promised connection speed.
You'll also pay $29.95 for shipping, but you'll get a return label in the box. The shipping fee is waived if your trip is longer than seven days. Insurance is $3.95 per day, but I'd recommend it since replacing a lost MiFi can cost $800. There's also a pricey $3-per-day fee for the extra battery (a flat rate would be more agreeable) and you'll be subject to additional fees if you're late in returning the device or if you cancel early.