It depends on the 3-D course for me. Formalized 3-D is fun, but not much more than practice with my hunting gear (I only shoot my hunting setup, whatever the target). Stumping is great for mixing up distances etc. and I would consider 2nd best overall.
However, the way we often set up targets on our 3-D courses and the informal nature of many "after competition" rounds, it is definitely better than roving for me. The one vital component missing in stump shooting is the necessity to pick a small spot on a target with little or no visible cues to aid in that targeting. Usually when stumping we are picking out a tiny target...but it's really not that difficult to hit compared to a blank 3-D profile, imho, because it's so easy to focus on the tiny spot presented. Much harder for me to pick that spot and burn it in on a 3-D, even though I know where the kill/scoring zones are. Like Mocsin, we often take turns picking out where to shoot and from forced positions...it definitely makes me a better hunting shot than either formal 3-D or stumping. YMMV, but either way, you'll have a great time watching the arrows fly.