I was paying $60/mo for a verizon pcmcia card, but discontinued it when I got a Blackberry Bold. The fee for data on the blackberry including tethering was... $60! So for the same price I could have access on my laptop, or access on my laptop AND a blackberry. Seemed like a no-brainer.
There is no noticeable difference in performance (although AT&T uses a different technology than Verizon) - although I would say Verizon's 3G network footprint is larger.