I rode a bit in those towns when I was living in S'ville. There are some nice spots in Billerica (also worked there, so got to see a bit more of it). The main roads through Wilmington and Tewskbury were nothing to write home about. On the other hand, Tewksbury does have Donna's donuts. If you're road-riding, when you've got Concord and Carlisle or the seashore so close, why bother with other places?
There is that unmentionable mountain biking spot over that way, but I think people like keeping it on the DL or something.