Can lefties use flex nibs and obtain conistent results?

Now there's a good question, I'd be interested in hearing the answers. My experience would be "no". I did try and really got nowhere, so sold the pen on instead.

By flex nibs do you mean dip pen flex nibs?

Depends on whether you are an overwriter or an underwriter. If you're an underwriter, I've been told by a true writing expert that you are actually in the best postition to do Copperplate. Haven't tried it yet, but I will one day.

Have a look at this post and the discussion in the thread.

as a temporary underwriting leftie, until my right shoulder blade injury heals, i find that leftie flexy writing is every bit as easy and controllable as rightie writing.

I hope you've had a chance to check out the link Antonios posted and look at his pen-hold sketches. If you are an underwriter, you will be in an ideal position to render copperplate letters with a flex nib. (See the lower left sketch of the four done in blue.)

As Antonios shows, it's possible (but awkward IMO) to use a flex nib as an overwriter producing shaded writing.

You didn't tell us if you are an overwriter or underwriter, or whether you had shaded writing in mind, but you can always ask more questions if we didn't cover what you hoped to find out.

The general answer to your question, as stated, is YES!

HTH,
Ann