Depends too if you decide to travel internationally. With Sprint, you would have to carry an additional GSM handset, and pay much higher rates than Cingular or T-Mobile.

My company uses Nextel, and as a business customer, they provide us with excellent customer service (I manage the relationship with Nextel). Much better than any other service that I have dealt with (it will be interesting to see what happens now with Sprint). I wouldn't recommend Nextel for an individual subscriber, however. I'm happy with T-Mobile for my personal phone account.