It looks like T-Mobile is going to cap the heavy users with a 1GB monthly bandwidth of usage for the high speed access. That means a less congested network; once you weed out the "gotta watch YouTube any chance I get" crowd. I think the mostly for entertainment user will not like the limited access and reduced speed after 1GB. I feel that joe business man mary sensible user will love the G1.
Most users will love the G1 not because of it's similarity with IPhone, but because of it's difference. That difference is the Android software and Google itself. Everyone that knows Google knows that Google is pretty successful at it's pursuits. Google and T-Mobile might reduce the IPhone to "red haired step-child" status, especially if android is adopted by other carriers. Plus, do your search...the G1 can be unlocked.
Since IPhone opened everyone up to the idea of smartphones, T-Mobile doesn't have to sell the market on smartphones, they just have to say they are better; most of the brush clearing has already been done by Apple. Dovetail that with the fact that no one has seriously challenged IPhone in the smart phone market and we might see a warm welcome for a Google/Android/T-Mobile contender (if 1.5 million orders to date is not enough to suggest that).