That is a quite the question! I guess it all depends on what you actually use and like on/about your blackberry. The app store is pretty cool but you can get a lot of software for BB too, so nothing too special in that dept (yet).
I would have to say that the G1 beats BBs as far as web browsing hands down so if actually surf the internet from your phone you may want to give the G1 a try. I hate to say it but my BBs cameras took better pictures

. If you are one of the people who like the BB because its easy to navigate with one hand, then you might not be as happy with the G1, but there's not much of a learning curve as the software is much more intuitive than BB's. I don't know about the Curve 8900 model but my G1 has much louder and cleared sound quality than any of the BBs I have used in the past. I also use a lot of google apps so the G1 is a given in that dept.
Now, if you already know that you do not like the design and look of the phone you might want to look elsewhere, I mean who wants to walk around with a phone that they mumble under their breath about (...stupid chin piece...j/k)

. I actually like the chin piece because I do the whole hold my phone with the shoulder thing sometimes and it helps it stay in place (but I think I am in the minority on this one.)
That said you can always buy a G1 and if you don't like it you can exchange it before your exchange period expires.