I wasn't using them because my test machine uses an LCD display that was a trifle fuzzy to start with, scanlines look strange on it. However the machine I'm actually putting in my cab project has a 19" VGA (computer) monitor, and for that I use the "auto" setting for scanlines thru Direct3D - that setting, in theory, chooses the best scanline effect for the resolution, refresh rate, etc. on a game-by-game basis. So far, 90% of the time it has chosen 25% scanlines; I have my scanline intensity set for 40%, and it looks fine for all the games I have tried. Vertical games look a trifle strange since the lines go the wrong way, but what the hey, it's good enough that I can enjoy the game and feel like it might be a real machine, without the blurring that I got with a TV, which to me is all that matters.
I'm not at all surprised at the lack of votes or the fact that the general response has been "use an arcade monitor" or the like. Lots of the regulars here (and most of the responders are regulars) have several real machines in addition to their MAME machines, and tend towards authenticity. That's their right and opinion and I have a great deal of respect for them and their ideas. However, I feel that it's a disservice to any newbies that would read this thread to represent that as the general consensus. Personally I feel that in general absolute authenticity is unnecessary, as most people out there would be happy with the simulation.
I don't think anyone can make a decision about using/not using scanlines or about any other video/audio option without trying it themselves, as only they themselves can decide if it's good enough for their purposes. Try everything, pick what you are happy with... that's my recommendation.
