Ranked in these QuestionsQuestion Ranking
Pro Supports Mac, Linux, Windows
It works Mac OSX Yosemite. Windows and Linux 32bit and 64bit are both supported.
Pro Most user-friendly visual editor
Con Not free!
Free version is time limited. Quote: (From the second month, you'll have 5 days Free every month. Need more? Go Premium!)
Con Bootstrap only
Does not do anything else it seems. Cost nothing maybe, but time is money. You may not want to spend time learning a tool that'll only do Bootstrap. Unless perhaps if you plan on doing JUST Bootstrap.