I've done it for short amount of times, mainly when I've had a string of injuries and there is only like a Kenny Rogers on the WW.
Yeah, don't really go after setup guys like jam said, because most managers save them for hold situations. Go after a stud RP guy in a deep bullpen, someone who will give you great ratios without being a setup guy.
Kou is one guy that comes to mind from this year, but he's been pretty decent that he maybe gone in some leagues. Tyler Bucholz might be another guy, although if Fuentes gets traded, he'll either be a setup man or closer, which goes against you.
I don't know if you have IP limits, or transactions limit, but I've also tried streaming RP, with the general rule that 1IP means they won't pitch the next day, and 2 IP means they might not pitch for 2 days. And there are tons of RPs out there, so your options are better than streaming just starters. But this takes a lot of time, as you have to find someone who is actually good, hasn't pitched the day before, and has a good shot at pitching the day you have him.
But I haven't tried it for an extended period of time, mainly because it tends to get time consuming. For short spurts it can help you. But after awhile, you start to get lazy, and leave the RPs in without streaming, and it hurts you.