I think they're fine dogs.
Haven't ever had a purebred myself, but I've had mixes.
My nephew was bit by one, but I blame the owner since he'd apparently had problem dogs before in the past as well.
My brother actually owns a Jack Russell - terrible dog. Always ready to bite, even just at raised voices or if he wants to rough house and play? Yeah, dog needs put away or it'll try to rip the leg off the other person. Or if he's asleep or you go into his bedroom - attacked. To the point of drawing blood through jeans - though I wasn't the one bit.
And this behavior is catching - he got a chihuahua not too long ago and now that one bites as well.
Little dogs can be just as aggressive as the larger - just not quite as likely to send you to the hospital unless your a kid.