A Blackberry is great if you want an email device (as your first priority) and a phone second. They do both those functions pretty well. Don't know which BB you are considering, but my recommendation is to get the BB Pearl as it's cheap, well built and stable.
The Nokia N73 is a decent phone, though I found the OS a little slow (on my N91 which also uses S60 v3). Just my 2p/c worth.
At the end of the day, it depends on what you are going to use it for, though I would go for the BB Pearl every time. Had it for about 9 months (work) and it was a great device, very stable too.