Today marks the official national launch of T-Mobile's HotSpot @Home service, whereby subscribers can use their cell phones to roam between cellular and Wi-Fi networks for calls. Just who is this service for, and is it worth the $19.99 per month?

As with everything, the answer to that question depends on what type of cell phone user you are and what your specific needs are. First, though, some details.

In order to use this service, you need to have a phone with Wi-Fi on board. Unfortunately, however, that doesn't mean a smartphone. Right now the service works with only two models, the Nokia 6086 and the Samsung t409. Both are basic feature phones bereft of many of the advanced features that sophisticated users are beginning to crave in their devices. You also need a Wi-Fi router. T-Mobile is selling its own branded routers (and gently suggests that you buy theirs), but the service works with most unlocked, open Wi-Fi networks. Oh, and of course the HotSpot @Home service plan, which runs $19.99 per month for individuals and $29.99 for families. (There is a temporary $10 discount on the pricing, lowering the costs to $9.99 and $19.99, respectively.) Once you have those three components, you're all set.

In my tests of the service, it worked fairly well. First off, there's no configuration necessary. Any dummy can use it right out of the box. T-Mobile definitely made it easy to use. Calls placed in cellular zones transfer over to Wi-Fi networks (such as T-Mobile HotSpots) about 20 seconds after walking into the hotspot and vice versa. There is no discernible signal to either the caller or callee that the call has transferred from one network to the other. It was truly seamless. Only once did the service drop a call when walking away from a Wi-Fi hotspot. Call quality was all over the place, though. I experienced both good and poor quality on both T-Mobile's GSM network as well as the hotspots.

So, who is this service really for? Well, not the enterprise. The range for the hotspot coverage for calling was just enough to make calls from inside my home. The second I stepped outside, the calls transferred to the cellular network. While you might be able to get away with sticking a router in your office for in-office calls, I have a funny feeling your IT admins might eventually object. And data calls, you know, firing up the good old Internet, still connect through T-Mobile's Edge network, and not the Wi-Fi system.

Consumers, however, can definitely make good use of this service, especially those in poor coverage areas. (There's a reason T-Mobile called it HotSpot "@Home.") The service extends T-Mobile's networks into the home so users can make calls from their cell phones easily. And aside from enhanced coverage, the ability to make "unlimited" calls when on the Wi-Fi network is certainly appealing to those of you who gab like mad. But in the end, that becomes a budgetary decision. T-Mobile's voice plans are not exactly stingy. The only potential enterprise use of this service is for the SoHo subscriber who also happens to be located in a poor coverage area and makes many phone calls per month.

The real drawback is the lack of available handsets. As more and more enterprise users switch to using smartphones, the two entry-level models that work with the service can be a real deal-breaker. Let's keep our fingers crossed that T-Mobile introduces more phones that are compatible with the service, especially some of its smartphones and enterprise-class devices, such as the HTC Touch.