I own one.
I find, or have found that it does help. Especially on courses that you have never played before. It removes most doubt on how to play shots. I consider it a tool, and not some sort of gimmick.
Just ask yourself how many times have you played a course for the first time, and after your done, count up how many shots you threw away because of uncertainty. Most of the guys I play with have rangefinders. We play very fast. In fact as we walk, you can carry on a conversation while you shoot yardages.
And they do not slow play down at all. They might even speed play up. Instead of looking for sprinkler caps, then pacing them off, and then consulting with a pinsheet (if you have one), then triangulating on how far away you are from the spinkler cap, then thinking if you have to add or subtract yardage because of wind or elevations, you just have to point and shoot to get your yardage.
As far as tournaments, I like to use them because a lot of the tournaments I play in are on courses that I haven't played before, or haven't played in a while. They are extremely helpful in that aspect. You can swing the club with a lot more confidence knowing that you have the correct yardage. Also, when your on the tee and you have a bunker out in the fairway that you are not sure on whether you can carry it or not, it'll remove that doubt.
But the great thing about golf. Rangefinders are not required. If you don't want one, you don't have to buy one.