I'm a Sprint subscriber but thinking about switching over to T-Mobile prepaid once Nexus 4 comes out. I have a pre-paid T-Mobile phone for work and I did a signal test yesterday on my way home (GWB to Rt. 4, then New Bridge Rd in Teaneck to Blvd in New Milford, and onto Grant Ave and Oradell Ave in Oradell). The signal strength and coverage were pretty decent; obviously better on Rt. 4 than local roads. There were, however, a couple pockets of 1 bar or even no bar along Blvd in New Milford.
Not sure where you live in Bergen County. It might help if you provide a bit more info on your neighborhood so other folks can help you out.
Good luck.