I have a S&W 410 which has a combination safety/decocker. I agree with Zander about having a way to drop the hammer without using the trigger. So in that sense, I like it.
I also use this same pistol to shoot IPSC (production class) matches. (OK, "match", but I'm shooting my second one next Saturday.) The downside to the safety/decocker is when it comes to the end of a stage and it's time to safety and clear the pistol. After the stage, I put on the safety (dropping the hammer) and then drop the mag and show clear to the range officer. The next step is to drop the hammer (already down) which must be done by pulling the trigger. S&W pistols have a magazine safety, that means I have to insert an empty magazine, take the pistol off safety, pull the trigger, then resafety the pistol and drop the magazine. It's a bit of a production.
Another gray area is movement during a match. The rules say the safety "should" be on during movement. (8.5, 14th edition) If I apply the safety, the hammer decocks, and the RO might think I was moving with my finger on the trigger and accidentally pulled the trigger during movement. However, the instructor at the IPSC safety class I took said the safety was optional during movement. (An assistant at the class was one of the RO's during my first match and said the same thing. I am not sure where I stand on this issue.)
There's my 2 bits worth.
Jhisaac