I see that almost everyone prefers enemas to laxatives. But INeedasudsE and Gummimann bring up a good points, which I'd like to expound on. I don't use them to treat constipation - I have a fetish related to being forced to go to the bathroom, and my wife gives them to me as part of our BSDM play (bless her). If we only have an hour or two to play, and enema is the perfect solution (no pun intended). It works fast and you can pretty much bank on it doing what it's supposed to do within an hour (I generally use two to four disposable Fleet enemas to guarantee success).
One of the most enjoyable aspects of our e-play is the ANTICIPATION - the part after she administers the enema(s), until I start to feel it/them working. That's the part that drives me wild. We rarely both have two days off work with nothing planned during those two days. But once in a while, when we do, we'll do the laxative thing: a dose of Ex-Lax slightly among the recommended dosage. It actually takes about a day and a half to work for me. And during that day and a half, the adrenaline that fuels my klimaphilia and my sex drive floods through my insides, sending my psyche into a frenzy.
We have a general idea of when it's going to hit, but we don't know exactly when, which is the exciting part. She'll put a diaper on me and make me go out and run errands with her...all the while, slyly grinning at me because she knows my secret and no one else does. She'll wait for the first indication that I'm feeling the effects, and then we'll start to head home, because she knows I'm afraid of having an accident in public.
So - in that context - I prefer laxatives to enemas, for obvious reasons.
There's another advantage to laxatives that I've found. When they do begin to work, the first stuff that comes out is hard and solid, more like a normal b.m., which stimulates my prostate and produces that anal-euphoric sensation of having a really great, life altering b.m.
I expect there must be others out there who have used laxatives for this purpose and prefer them to enemas for the same reasons.
At 51, I just discovered castor oil for the first time (where have I been?), which occupies the middle ground between enemas and laxatives, tending to work (for me) in about three hours. But when it does, what comes out is pretty much liquid, so I don't get to enjoy the prostate stimulation I do with laxatives. :-)