I am currently with Sprint and have an I300 and I am totally unhappy with the service both technically and as far as customer service goes. If the Crackberry they offer will be using the current Sprint network then I would steer away. With the Cingular/ATT merger I don't think you would be able to find a better network, IMHO.
The services are already merged/merging. As far the final hooray on paper, I have heard it should be by the first quarter of next year. But I have a friend that is with AT&T and his phone is already going between networks. He showed me the other day where his Blackberry from AT&T said Cingular on the screen. And it swaps back and forth.
I am buying mine on my own, but my employer is covering my Data plan, since we currently all use the lil 900s. I have just become more and more unhappy with my Sprint service and then when I saw Cingular had a BB I had a light bulb go off in my head. I could merge my phone and my pager into one item and get rid of the bat belt look.