The phone, expected to be the first of many to use the Android operating system, was largely designed by Google, and was built by HTC of Taiwan. It will be sold in the U.S. starting next month by T-Mobile, for $179 with a two-year contract.

Here are some first impressions of the G1, based on some experience with a prototype. This isn’t a full review; that will come later, when I’ve had a chance to use a more finished device.

Most importantly, the G1 complements its touch screen with a physical keyboard, the lack of which has made the iPhone a non-starter for some users. The G1’s keyboard is revealed when you slide open its screen. The keys are a bit flat, and you have to reach your right thumb around a bulging portion of the phone’s body to type, but it’s a real keyboard. And there’s also a BlackBerry-like trackball that supplements the touch screen navigation. I found typing on this keyboard to be OK, but not great.

A second big feature, or limitation, of the G1 — depending on your point of view — is that it is tightly tied to Google’s web-based email, contacts and calendar programs. In fact, you must have a Google (GOOG) account to use the phone, and can only synchronize the phone’s calendar and address book with Google online services. Unlike the iPhone, it doesn’t work with Microsoft Exchange, and it can’t physically be synced with a PC-based calendar or contacts program, like Microsoft Outlook.

So, if your world already revolves around Google services, you may find that the G1 fits like a glove. If not, you may be disappointed.

Also, like the iPhone, the G1 has a download service for third-party programs, called Market. I downloaded a couple of simple Market programs and they worked fine.

The G1 won’t win any beauty contests with its Apple (AAPL) rival. It’s stubby and chunky, nearly 30% thicker and almost 20% heavier than the iPhone. It’s a bit narrower — more like a standard phone than a “smart phone” — and longer, but has a somewhat smaller screen.

Still, it feels pretty good in the hand when closed, although I found it more awkward when opened.

But the software is slick. Programs appear in a virtual drawer you slide open via a tab at the bottom of the screen, and notifications of new messages and the like can be read by sliding the top bar of the screen down. The screen and software were quick and responsive.

The web browser is based on the same open-source technology as the iPhone’s, but works differently. You can view a portion of a page, and use a zoom control and finger-dragging to see the rest, or you can view the whole page in miniature, as on the iPhone. In the latter mode, however, you can’t simply use Apple’s technique of tapping or “pinching” to zoom in on a portion of a page. You must move around a virtual lens to pick out a part of the page on which to focus.

There are two email programs: one for Google’s Gmail, another for all other email services. There’s an instant messaging program, that works with multiple services — not just Google’s. And, as on the iPhone, there are programs for using Google Maps and Google’s YouTube video service. The G1’s Google Maps program has a feature lacking in the iPhone version: photographic street views of some locations.

The G1 has a couple of other things the iPhone omits: copy and paste functionality and a so-called MMS program, which sends photos to other phones without using email. Its camera is higher-resolution than the iPhone’s, but, like Apple’s, doesn’t record video.

It also gives you far more flexibility in organizing your desktop, or home screen, than the iPhone, or almost any phone I’ve seen. In addition to placing icons for programs there, you can place everything from individual contacts, music playlists, folders, web pages, and more.

The G1’s multimedia capabilities are less polished and complete than the iPhone’s. There’s a very basic music player, and a built-in version of Amazon’s MP3 download service that works fine. But the G1 lacks a built-in video player — you have to download one from the third-party software store. Also, you cannot use standard stereo headphones with the G1. You need special ones, or an adapter.

And it lacks the iPhone’s ability to change the orientation of a web page or photo by just turning the phone. You also can’t move through groups of photos by just “flicking,” as on the iPhone.

The G1 also has much less memory than the iPhone. The base $199 iPhone comes with 8 gigabytes sealed in, but the G1 comes with just a 1 gigabyte memory card. Its maximum memory, if you buy a bigger card, is 8 gigabytes, while the iPhone can be purchased (for $299) with twice that.

T-Mobile is claiming similar talk time to that of the iPhone, but, unlike Apple’s product, the G1 has a removable battery.

Finally, a word about networks. In the U.S., the G1 will initially only be available on T-Mobile, whose high-speed 3G network will be up and running in many fewer cities than those of its larger rivals, AT&T (T) and Verizon (VZ). Like the iPhone, the G1 does have Wi-Fi, Bluetooth and GPS.

In sum, the G1 is a powerful, versatile device which will offer users a real alternative in the new handheld computing category the iPhone has occupied alone.

The Biggest Pluses

Speed: Because the biggest problem with the original iPhone was the slow AT&T (T) network, moving the device to the much faster 3G network, while no surprise, will have a huge impact. This is especially true since data already show that it is by far the most heavily used smart phone for Web surfing. It means that you won’t have to search for a Wi-Fi network to do decent Web surfing.

Price: Less than one year after it was introduced at a hefty $599 price, the 8 GB model of the iPhone will now be two-thirds cheaper, at just $199.

Third-party programs: Until now, in order to get third-party programs on an iPhone, you had to hack it. Now, there will be hundreds of them coming soon, and the handful that were demonstrated by Apple looked impressive, from blogging-on-the-go, to news and sports, to games and even medical software.

The Biggest Minuses

Still locked to one carrier: Even though the greater speed is a huge deal, it only matters if you have good AT&T reception. If AT&T’s coverage in your area is poor, the iPhone is still a bad choice, because Apple, unlike its competitors, doesn’t sell it through multiple carriers in one market.

Still missing some features: They haven’t added a real way to cut and paste, or to save files, other than emailed photos. And there still isn’t any MMS capability–the ability to instantly send or receive media files over the phone network without resorting to email. There’s also no instant-messaging program, though third-party developers are likely to offer these.

Still has a wimpy camera: The original iPhone camera was OK, but, given the phone’s brilliant screen and tight integration with computer photo software, it could stand to be better. It isn’t in this new model.