@Ivanov: Part of the ability of Ensis Exorcizans is to counter magic. It's probably not as potent as Asuna's natural cancel, but it's still there. At the same time, it COULD just be an extention of Asuna's power and thus be useless in Negi's hands, but he should have been able to find that kind of thing out during his hyperbolic-time-chamber training.
@death_incarnate: His isn't really the best considering he can only really use Konoka's, Setsuna's, Asuna's(?), and maybe Nodoka's (as Heartlessfang listed), but her's would require those two artifacts she went out of her way to find.