I've had my Nokia E61 for about 2 months now. There's a number of reasons I chose it over a Blackberry. First of all, it has better functionality than blackberries. I get Wifi, Bluetooth, 3G, infra-red and usb connectivity out of the box. The expansion possibilities are endless. You can even get things like GPS on for it. It's all supported out of the box.
Besides these things the screen is much better quality than you get on a Blackberry. Its high-res with 16 million colors. Looks great. The browser on the Nokia is also better than the one on the Blackberry.
If you are considering this Nokia E61, I HIGHLY recommend an unlimited internet access plan of some sort to allow you to check your email and browse the net on the go. If you're anything like me you're going to get addicted and'll end up with very high bills if you use pay as you go.
I recommend checking for some reviews of the E61 before you buy.