In summary
Lacking features like VPN and Exchange Server connectivity, T-Mobile G1 does not pass muster as an enterprise device, but that's a limitation that I expect will be overcome by Google and open source developers much sooner than the year it took Apple to turn iPhone into the enterprise-friendly iPhone 2.0.
T-Mobile G1's run of the mill build quality is offset by a 12-month parts and labor warranty managed directly by the manufacturer, HTC. Shipping in both directions is covered under the warranty. While the G1 is not yet as well suited to corporate fleet deployment as BlackBerry or iPhone, it is a solid handset for professionals, highly demanding consumers, and developers.
Uptake among buyers and coders alike is already strong, with T-Mobile's initial run of 1.5 million devices wiped out by pre-orders. I expect T-Mobile G1 to compete in unit sales, if not exceed iPhone. G1 is cheaper by about US$120, and since T-Mobile controls the pricing of both the phone and coverage plans, there's a lot of wiggle room on that price. I predict a price war between Apple/AT&T and Google/T-Mobile that will bring the price of both handsets down in a few months.
Even at its full initial price of US$179 with a two-year 3G contract, T-Mobile G1 shines as the best available handset for consumer and individual professionals this side of Windows Mobile, and G1 is immeasurably easier to use and write code for. I'm confident that currently-missing features like Office and PDF document viewing will appear in Google's Android Market soon enough to make early purchase of T-Mobile G1 a low-risk exercise. If you don't need VPN or Exchange e-mail, and you can get a T-Mobile G1, get one.