I'll be honest, in terms of wanting a relationship, I probably wouldn't ever use one of these apps. Mainly because everyone on them seems to be in general agreement that they're used for "hook-ups" and nothing more. It's impossibly frustrating trying to get a serious conversation out of somebody who just wants in your pants- and even when you get one, it doesn't mean they're open to anything serious.
Now if we're talking just to fuck? Hell yeah.
I love apps like Tinder and Hot or Not to find hookups. Most of my matches I only flirt with, and never actually meet- but it's nice to know the option is there. I extraordinarily love the fact that I don't have to leave my house just to meet someone new near me to plan a date with. It's like online dating without the worry that they're halfway across the world.... and you get to fuck!
These apps are especially good for traveling. Whenever I visit another city than my own- I immediately download Tinder. Not even just for the hookups but for local advice on places to eat, things to do, and sometimes- a great club that's open late. When traveling, I don't find I have much spare time between eating, finding my way, and checking of my to-see list; and I certainly don't want to get drunk in a bar and wakeup with some stranger next to me (just because, that's really dangerous, and you should never follow a stranger to their home when you're intoxicated). Dating apps make it easy for me to get a "pleasure date" into my schedule without worrying if I'll find someone.