I have owned an HD500 for the last year and yes you could add an effect on the fly but I wouldn't recommend it. I have a standard set of effects that I use, NG, OD, Comp, Delay, Rev that I use that I have dialed into a sound that I like. I then use that set as a template if I need something different for a song, but I'm making those changes at home with a laptop and not on the unit at the gig. Yes you can make changes if you really need to on the unit, but it's not a simple process.

If you need to adjust quickly on the fly, then something like the Boss ME-70 is probably a better ME unit.