since Bec Tel went Belly Up 2nd last december I use the USB Stick from Claro, not the prepaid versions, I have it on contract, and since then I am very happy with it, signal is great all around, at home the same than on the Road.
I did not use Orange but hear good stuff about their product, too.
Tricom I know several friends who have it, all use it only for once in a while e-mail checks, not for work, and even for that minor use it is in their opinion not worth the cheaper rate. the problem with the Tricom Wimax contracts here is that you have to travel to their mainoffice in Santo Domingo in case you want to cancel your contract, alone that feature would be for Me reason enough to not to deal with a particular Company.
a friend of mine living next door here has the USB syick from Viva and is also very happy with the reliability of the service.
Mike