I've always been a proponent of the idea that any story-type can be written well. The last one that came up was Tenth Walkers in LotR, and I spent a little while proving that one.
I don't think I've done OC/canon (well, technically I have, since Dafydd == Maglor, but that's rather different), but I have written a romance between two unconnected canon characters, which hits many of the same points: The Horn and the Harp.
But yeah, it basically comes down to the same questions as any other story. Is the canon in-character? Is the OC believable? Does the plot make sense in light of both the setting and the characters? Then you have a workable story. ('Good', obviously, requires things like spelling and nice writing, as well, but at the least this wouldn't be bad).
Of course, some characters are more difficult to pair off than others. Finding a girlfriend for Captain Kirk shouldn't be hard, though I wouldn't expect it to last long. Breaking up Anakin and Padme would be a lot harder, because Ani is an obsessive lunatic. You could probably get Padme to leave him - she was basically doing so when he killed her, so shuffle events around, make his darkness more obvious, and you could pull it off - but that would likely just flip him into full-on Vader mode early, making a relationship with an OC unlikely.
("But you said any story-" Yes, and I'll stand by that. If challenged, I'd go with either killing Padme, or having her take a stand against the Jedi for political reasons. The former, if done early enough, would give Ani time to back down from the Dark Side, briefly; the latter could drive a wedge between them, with him taking her actions as a personal insult. Since this would be pre-Vader, he wouldn't actually strangle her, so it could work)
hS